diff --git a/assets/src/components/phone/style.css b/assets/src/components/phone/style.css
index de599f27147..8ad1355cb4a 100644
--- a/assets/src/components/phone/style.css
+++ b/assets/src/components/phone/style.css
@@ -4,7 +4,7 @@
 	display: flex;
 	flex-direction: column;
 	margin-bottom: 1rem;
-	max-height: 413px;
+	height: 413px;
 	min-height: 200px;
 	padding: 12px;
 	position: relative;
@@ -25,5 +25,8 @@
 }
 
 .phone-inner {
+	display: flex;
+	background-color: var(--color-gray-medium);
+	flex-grow: 1;
 	overflow: hidden;
 }
diff --git a/assets/src/components/reader-themes-context-provider/__mocks__/index.js b/assets/src/components/reader-themes-context-provider/__mocks__/index.js
new file mode 100644
index 00000000000..7f1509cbacd
--- /dev/null
+++ b/assets/src/components/reader-themes-context-provider/__mocks__/index.js
@@ -0,0 +1,34 @@
+/**
+ * External dependencies
+ */
+import PropTypes from 'prop-types';
+
+/**
+ * WordPress dependencies
+ */
+import { createContext } from '@wordpress/element';
+
+export const ReaderThemes = createContext();
+
+/**
+ * MOCK.
+ *
+ * @param {Object} props
+ * @param {any} props.children Component children.
+ * @param {boolean} props.downloadingTheme Whether downloading a theme or not.
+ */
+export function ReaderThemesContextProvider( { children, downloadingTheme = false } ) {
+	return (
+		<ReaderThemes.Provider value={
+			{
+				downloadingTheme,
+			}
+		}>
+			{ children }
+		</ReaderThemes.Provider>
+	);
+}
+ReaderThemesContextProvider.propTypes = {
+	children: PropTypes.any,
+	downloadingTheme: PropTypes.bool,
+};
diff --git a/assets/src/components/reader-themes-context-provider/index.js b/assets/src/components/reader-themes-context-provider/index.js
index c491bc669bb..b39f7041f88 100644
--- a/assets/src/components/reader-themes-context-provider/index.js
+++ b/assets/src/components/reader-themes-context-provider/index.js
@@ -9,6 +9,7 @@ import { __ } from '@wordpress/i18n';
  * External dependencies
  */
 import PropTypes from 'prop-types';
+import { USING_FALLBACK_READER_THEME, LEGACY_THEME_SLUG } from 'amp-settings';
 
 /**
  * Internal dependencies
@@ -80,16 +81,20 @@ export function ReaderThemesContextProvider( { wpAjaxUrl, children, currentTheme
 	const [ downloadingThemeError, setDownloadingThemeError ] = useState( null );
 
 	/**
-	 * If the currently selected theme is unavailable and not installable, or the current theme is the active theme,
-	 * unset the reader theme option.
+	 * If the currently selected theme is not installable, is the active theme, or unavailable for selection, set the
+	 * Reader theme to AMP Legacy.
 	 */
 	useEffect( () => {
 		if ( themeWasOverridden ) { // Only do this once.
 			return;
 		}
 
-		if ( selectedTheme.availability === 'non-installable' || originalSelectedTheme.availability === 'active' ) {
-			updateOptions( { reader_theme: 'legacy' } );
+		if (
+			selectedTheme.availability === 'non-installable' ||
+			originalSelectedTheme.availability === 'active' ||
+			USING_FALLBACK_READER_THEME
+		) {
+			updateOptions( { reader_theme: LEGACY_THEME_SLUG } );
 			setThemeWasOverridden( true );
 		}
 	}, [ originalSelectedTheme.availability, selectedTheme.availability, themeWasOverridden, updateOptions ] );
diff --git a/assets/src/components/svg/desktop-icon.svg b/assets/src/components/svg/desktop-icon.svg
new file mode 100644
index 00000000000..a8c323ec548
--- /dev/null
+++ b/assets/src/components/svg/desktop-icon.svg
@@ -0,0 +1,49 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 236 178">
+	<defs>
+		<style>
+			.d-cls-2,.d-cls-5{fill:#fff;}.d-cls-3{fill:#c4c4c4;}.d-cls-4{fill:#bababa;}.d-cls-5{fill-opacity:0.1;}.d-cls-6{stroke:#fff;stroke-width:2px;}.d-cls-7{mask:url(#desktop-mask);}.d-cls-8{fill:#e7e7e7;}
+		</style>
+		<mask id="desktop-mask" x="-0.31" y="84.35" width="236.31" height="18.69">
+			<g>
+				<rect class="d-cls-1" x="-0.25" y="85.1" width="236.16" height="17.18" transform="translate(-0.6 0.75) rotate(-0.37)"/>
+			</g>
+		</mask>
+	</defs>
+	<g>
+		<g>
+			<rect class="d-cls-2" width="236" height="178"/>
+			<rect class="d-cls-3" width="236" height="86"/>
+			<polygon class="d-cls-4" points="234.13 86 157.23 24.1 93.83 64.38 77.44 44.56 5.84 86 234.13 86"/>
+			<circle class="d-cls-5" cx="31.95" cy="30.81" r="12.68"/>
+			<path class="d-cls-2" d="M42.15,10.37l16.46-.1a2.28,2.28,0,0,1,2.3,2.26h0a2.27,2.27,0,0,1-2.27,2.3l-16.46.11a2.28,2.28,0,0,1-2.3-2.27h0A2.29,2.29,0,0,1,42.15,10.37Z"/>
+			<path class="d-cls-2" d="M104.06,11.12l9.14-.06a1.13,1.13,0,0,1,1.15,1.13h0a1.13,1.13,0,0,1-1.13,1.15l-9.15.06a1.14,1.14,0,0,1-1.15-1.13h0A1.15,1.15,0,0,1,104.06,11.12Z"/>
+			<path class="d-cls-2" d="M120.06,11,129.2,11a1.13,1.13,0,0,1,1.15,1.13h0a1.13,1.13,0,0,1-1.13,1.15l-9.15.06a1.15,1.15,0,0,1-1.15-1.14h0A1.14,1.14,0,0,1,120.06,11Z"/>
+			<path class="d-cls-2" d="M136.06,10.91l9.14-.06A1.14,1.14,0,0,1,146.35,12h0a1.14,1.14,0,0,1-1.13,1.15l-9.15,0a1.13,1.13,0,0,1-1.15-1.13h0A1.14,1.14,0,0,1,136.06,10.91Z"/>
+			<path class="d-cls-2" d="M152.06,10.81l9.14-.06a1.14,1.14,0,0,1,1.15,1.13h0A1.13,1.13,0,0,1,161.22,13l-9.15.06A1.14,1.14,0,0,1,150.92,12h0A1.15,1.15,0,0,1,152.06,10.81Z"/>
+			<path class="d-cls-6" d="M177.22,10.5l16.22-.1a1.29,1.29,0,0,1,1.29,1.28h0A1.3,1.3,0,0,1,193.45,13l-16.22.1a1.27,1.27,0,0,1-1.29-1.27h0A1.29,1.29,0,0,1,177.22,10.5Z"/>
+			<rect class="d-cls-2" x="49.91" y="36.84" width="136.07" height="9.14" rx="4.57" transform="translate(-0.26 0.76) rotate(-0.37)"/>
+			<path class="d-cls-2" d="M91.41,52.71l53.22-.35a1.15,1.15,0,0,1,1.15,1.14h0a1.15,1.15,0,0,1-1.14,1.15L91.43,55a1.13,1.13,0,0,1-1.15-1.13h0A1.13,1.13,0,0,1,91.41,52.71Z"/>
+			<path class="d-cls-2" d="M103.2,58.05l29.71-.19A1.13,1.13,0,0,1,134.06,59h0a1.14,1.14,0,0,1-1.13,1.15l-29.72.19a1.15,1.15,0,0,1-1.15-1.13h0A1.15,1.15,0,0,1,103.2,58.05Z"/>
+			<path class="d-cls-3" d="M103.63,124.76l29.71-.19a1.15,1.15,0,0,1,1.15,1.14h0a1.15,1.15,0,0,1-1.14,1.15l-29.71.19a1.15,1.15,0,0,1-1.15-1.14h0A1.15,1.15,0,0,1,103.63,124.76Z"/>
+			<g class="d-cls-7">
+				<path class="d-cls-4" d="M0,86v10.9c23.41,2,66.54,3.17,115.89,2.86,52.52-.34,98-2.26,120.11-4.79V86Z"/>
+			</g>
+			<path class="d-cls-2" d="M100,74.92l36.55-.24a4.58,4.58,0,0,1,4.6,4.54h0a4.58,4.58,0,0,1-4.54,4.6l-36.55.24a4.58,4.58,0,0,1-4.6-4.54h0A4.58,4.58,0,0,1,100,74.92Z"/>
+			<path class="d-cls-3" d="M102.84,78.22,132.56,78a1.14,1.14,0,0,1,1.15,1.13h0a1.15,1.15,0,0,1-1.14,1.15l-29.71.19a1.13,1.13,0,0,1-1.15-1.13h0A1.14,1.14,0,0,1,102.84,78.22Z"/>
+			<path class="d-cls-3" d="M86.49,115.59l64-.41a2.28,2.28,0,0,1,2.3,2.26h0a2.28,2.28,0,0,1-2.27,2.3l-64,.41a2.26,2.26,0,0,1-2.29-2.26h0A2.27,2.27,0,0,1,86.49,115.59Z"/>
+			<rect class="d-cls-8" x="43.33" y="138.39" width="41.12" height="31.71" rx="10" transform="translate(-0.99 0.41) rotate(-0.37)"/>
+			<path class="d-cls-3" d="M59.38,154.51,49.13,170.2l31.73-.2L70.17,147.3l-8.54,14.35Z"/>
+			<circle class="d-cls-2" cx="53.22" cy="147.12" r="4.77"/>
+			<rect class="d-cls-8" x="98.15" y="138.04" width="41.12" height="31.71" rx="10" transform="translate(-0.99 0.77) rotate(-0.37)"/>
+			<path class="d-cls-3" d="M114.2,154.17,104,169.86l31.73-.21L125,147l-8.54,14.35Z"/>
+			<circle class="d-cls-2" cx="108.04" cy="146.77" r="4.77"/>
+			<rect class="d-cls-8" x="152.98" y="137.69" width="41.12" height="31.71" rx="10" transform="translate(-0.98 1.12) rotate(-0.37)"/>
+			<path class="d-cls-3" d="M169,153.82,158.77,169.5l31.73-.2-10.69-22.7L171.28,161Z"/>
+			<circle class="d-cls-2" cx="162.86" cy="146.42" r="4.77"/>
+			<path class="d-cls-3" d="M212.18,60l16.68.08a2.32,2.32,0,0,1,2.31,2.32h0a2.33,2.33,0,0,1-2.33,2.31l-16.68-.08a2.3,2.3,0,0,1-2.3-2.32h0A2.31,2.31,0,0,1,212.18,60Z"/>
+			<path class="d-cls-3" d="M210.38,50,215,50a.47.47,0,0,1,.46.47h0a.47.47,0,0,1-.47.46l-4.63,0a.47.47,0,0,1-.46-.47h0A.47.47,0,0,1,210.38,50Z"/>
+			<path class="d-cls-3" d="M210.37,51.81l4.63,0a.47.47,0,0,1,.46.47h0a.46.46,0,0,1-.47.46l-4.63,0a.47.47,0,0,1-.46-.47h0A.47.47,0,0,1,210.37,51.81Z"/>
+			<path class="d-cls-3" d="M210.36,53.67l4.63,0a.47.47,0,0,1,.46.46h0a.46.46,0,0,1-.46.46l-4.63,0a.45.45,0,0,1-.46-.46h0A.45.45,0,0,1,210.36,53.67Z"/>
+		</g>
+	</g>
+</svg>
diff --git a/assets/src/components/svg/mobile-icon.svg b/assets/src/components/svg/mobile-icon.svg
new file mode 100644
index 00000000000..ed894d55434
--- /dev/null
+++ b/assets/src/components/svg/mobile-icon.svg
@@ -0,0 +1,42 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 79 139">
+	<defs>
+		<style>
+			.cls-1{fill:#fff;}.cls-2{fill:#c4c4c4;}.cls-3{fill:none;stroke:#c4c4c4;stroke-width:2px;}.cls-4{fill:#e7e7e7;}
+		</style>
+	</defs>
+	<g>
+		<g>
+			<rect class="cls-1" x="-30.33" y="30.82" width="140.19" height="78.89" transform="translate(-30.69 109.72) rotate(-90)"/>
+			<path class="cls-2" d="M7.51,14.71l16.68.07a2.31,2.31,0,0,1,2.3,2.33h0a2.3,2.3,0,0,1-2.32,2.3L7.49,19.34A2.33,2.33,0,0,1,5.18,17h0A2.31,2.31,0,0,1,7.51,14.71Z"/>
+			<rect class="cls-2" x="32.11" y="-4.06" width="2.31" height="56.24" rx="1.16" transform="matrix(0, -1, 1, 0, 9.06, 57.22)"/>
+			<rect class="cls-2" x="18.84" y="58.89" width="4.63" height="32.47" rx="2.31" transform="translate(-54.06 95.95) rotate(-89.74)"/>
+			<path class="cls-2" d="M6.19,61.26l9.27,0a1.15,1.15,0,0,1,1.15,1.16h0a1.16,1.16,0,0,1-1.16,1.16l-9.27,0A1.15,1.15,0,0,1,5,62.41H5A1.15,1.15,0,0,1,6.19,61.26Z"/>
+			<path class="cls-2" d="M22.4,61.33l9.27,0a1.16,1.16,0,0,1,1.15,1.17h0a1.15,1.15,0,0,1-1.16,1.15l-9.27,0a1.16,1.16,0,0,1-1.15-1.16h0A1.16,1.16,0,0,1,22.4,61.33Z"/>
+			<path class="cls-2" d="M6.06,80.48l11,0a1.15,1.15,0,0,1,1.15,1.16h0a1.15,1.15,0,0,1-1.16,1.15l-11,0a1.16,1.16,0,0,1-1.16-1.16h0A1.16,1.16,0,0,1,6.06,80.48Z"/>
+			<path class="cls-2" d="M73.15,99.46l-19.56-.09a1.15,1.15,0,0,0-1.16,1.15h0a1.15,1.15,0,0,0,1.15,1.16l19.56.09a1.15,1.15,0,0,0,1.16-1.15h0A1.15,1.15,0,0,0,73.15,99.46Z"/>
+			<path class="cls-2" d="M6,91.43l11,0a1.15,1.15,0,0,1,1.15,1.16h0A1.15,1.15,0,0,1,17,93.79L6,93.74a1.16,1.16,0,0,1-1.16-1.16h0A1.16,1.16,0,0,1,6,91.43Z"/>
+			<path class="cls-2" d="M62.17,86.26l11,0a1.17,1.17,0,0,1,1.15,1.17h0a1.16,1.16,0,0,1-1.16,1.15l-11,0A1.16,1.16,0,0,1,61,87.41h0A1.16,1.16,0,0,1,62.17,86.26Z"/>
+			<path class="cls-2" d="M25.92,104.78l-20-.09a1.15,1.15,0,0,0-1.16,1.15h0A1.15,1.15,0,0,0,5.94,107l20,.09a1.15,1.15,0,0,0,1.16-1.15h0A1.15,1.15,0,0,0,25.92,104.78Z"/>
+			<path class="cls-2" d="M24,80.56l49.28.22a1.16,1.16,0,0,1,1.16,1.16h0a1.18,1.18,0,0,1-1.17,1.16l-49.28-.23a1.15,1.15,0,0,1-1.15-1.16h0A1.15,1.15,0,0,1,24,80.56Z"/>
+			<path class="cls-2" d="M47.71,99.34,6,99.15a1.15,1.15,0,0,0-1.16,1.15h0A1.17,1.17,0,0,0,6,101.47l41.74.18a1.16,1.16,0,0,0,1.17-1.15h0A1.16,1.16,0,0,0,47.71,99.34Z"/>
+			<path class="cls-2" d="M23.9,91.51l49.28.22a1.16,1.16,0,0,1,1.16,1.16h0a1.18,1.18,0,0,1-1.17,1.16l-49.28-.23a1.15,1.15,0,0,1-1.15-1.16h0A1.15,1.15,0,0,1,23.9,91.51Z"/>
+			<path class="cls-2" d="M6,86l49.28.22a1.15,1.15,0,0,1,1.15,1.16h0a1.16,1.16,0,0,1-1.16,1.16L6,88.32a1.15,1.15,0,0,1-1.15-1.16h0A1.15,1.15,0,0,1,6,86Z"/>
+			<path class="cls-2" d="M73.13,105l-41.79-.19A1.18,1.18,0,0,0,30.17,106h0a1.17,1.17,0,0,0,1.15,1.16l41.8.19a1.17,1.17,0,0,0,1.16-1.16h0A1.16,1.16,0,0,0,73.13,105Z"/>
+			<path class="cls-2" d="M38.62,61.41l9.26,0A1.16,1.16,0,0,1,49,62.61h0a1.16,1.16,0,0,1-1.17,1.15l-9.26,0a1.16,1.16,0,0,1-1.16-1.16h0A1.16,1.16,0,0,1,38.62,61.41Z"/>
+			<path class="cls-2" d="M54.83,61.48l9.27,0a1.15,1.15,0,0,1,1.15,1.16h0a1.15,1.15,0,0,1-1.16,1.15l-9.27,0a1.15,1.15,0,0,1-1.15-1.16h0A1.15,1.15,0,0,1,54.83,61.48Z"/>
+			<path class="cls-2" d="M5.7,4.65l4.63,0a.45.45,0,0,1,.46.46h0a.46.46,0,0,1-.46.46l-4.63,0a.46.46,0,0,1-.46-.46h0A.47.47,0,0,1,5.7,4.65Z"/>
+			<path class="cls-2" d="M5.69,6.51l4.63,0a.45.45,0,0,1,.46.46h0a.45.45,0,0,1-.46.46l-4.63,0A.46.46,0,0,1,5.23,7h0A.45.45,0,0,1,5.69,6.51Z"/>
+			<path class="cls-2" d="M5.69,8.36l4.63,0a.47.47,0,0,1,.46.46h0a.46.46,0,0,1-.47.46l-4.63,0a.47.47,0,0,1-.46-.46h0A.46.46,0,0,1,5.69,8.36Z"/>
+			<path class="cls-3" d="M56.12,5.88,72.55,6a1.31,1.31,0,0,1,1.31,1.32h0a1.31,1.31,0,0,1-1.32,1.31L56.11,8.51A1.32,1.32,0,0,1,54.8,7.19h0A1.3,1.3,0,0,1,56.12,5.88Z"/>
+			<rect class="cls-4" x="27.33" y="9.56" width="24.99" height="69.44" rx="10" transform="translate(-4.63 83.91) rotate(-89.74)"/>
+			<path class="cls-2" d="M29.36,44.42,9.54,56.64l60.56.28-20-18L33.54,50.07Z"/>
+			<circle class="cls-1" cx="14.39" cy="40.41" r="3.76"/>
+			<rect class="cls-4" x="8.41" y="108" width="24.99" height="32.41" rx="10" transform="matrix(0, -1, 1, 0, -103.39, 144.55)"/>
+			<path class="cls-2" d="M17.35,124.37,9.13,136.65l25,.11-8.23-18L19.06,130Z"/>
+			<circle class="cls-1" cx="12.56" cy="118.49" r="3.76"/>
+			<rect class="cls-4" x="45.64" y="108.16" width="24.99" height="32.41" rx="10" transform="translate(-66.49 181.94) rotate(-89.74)"/>
+			<path class="cls-2" d="M54.58,124.54l-8.22,12.27,25,.11-8.23-18-6.85,11.24Z"/>
+			<circle class="cls-1" cx="49.78" cy="118.66" r="3.76"/>
+		</g>
+	</g>
+</svg>
diff --git a/assets/src/components/theme-card/index.js b/assets/src/components/theme-card/index.js
index f6798c40f9d..4b29479b67a 100644
--- a/assets/src/components/theme-card/index.js
+++ b/assets/src/components/theme-card/index.js
@@ -14,10 +14,11 @@ import PropTypes from 'prop-types';
 /**
  * Internal dependencies
  */
+import './style.css';
+import MobileIcon from '../svg/mobile-icon.svg';
 import { Options } from '../options-context-provider';
 import { Selectable } from '../selectable';
 import { Phone } from '../phone';
-import './style.css';
 
 /**
  * A selectable card showing a theme in a list of themes.
@@ -48,14 +49,18 @@ export function ThemeCard( { description, ElementName = 'li', homepage, screensh
 		>
 			<label htmlFor={ id } className="theme-card__label">
 				<Phone>
-					<img
-						src={ screenshotUrl }
-						alt={ name }
-						height="2165"
-						width="1000"
-						loading="lazy"
-						decoding="async"
-					/>
+					{
+						screenshotUrl ? (
+							<img
+								src={ screenshotUrl }
+								alt={ name }
+								height="2165"
+								width="1000"
+								loading="lazy"
+								decoding="async"
+							/>
+						) : <MobileIcon style={ { width: '100%' } } />
+					}
 					{ disabled && (
 						<div className="theme-card__disabled-overlay">
 							{ __( 'Unavailable', 'amp' ) }
@@ -96,7 +101,7 @@ ThemeCard.propTypes = {
 	description: PropTypes.string.isRequired,
 	ElementName: PropTypes.string,
 	homepage: PropTypes.string.isRequired,
-	screenshotUrl: PropTypes.string.isRequired,
+	screenshotUrl: PropTypes.string,
 	slug: PropTypes.string.isRequired,
 	name: PropTypes.string.isRequired,
 	disabled: PropTypes.bool,
diff --git a/assets/src/components/theme-card/style.css b/assets/src/components/theme-card/style.css
index 2049d5651b9..d54dc8cf9d6 100644
--- a/assets/src/components/theme-card/style.css
+++ b/assets/src/components/theme-card/style.css
@@ -48,13 +48,10 @@
 	}
 }
 
-.theme-card .phone-inner {
-	max-height: 307px; /* Make all images uniform height. */
-}
-
 .theme-card img {
 	height: auto;
 	width: 100%;
+	margin: auto;
 }
 
 p.theme-card__description {
diff --git a/assets/src/onboarding-wizard/components/nav/index.js b/assets/src/onboarding-wizard/components/nav/index.js
index 40a0b3f63cf..8ef500981c2 100644
--- a/assets/src/onboarding-wizard/components/nav/index.js
+++ b/assets/src/onboarding-wizard/components/nav/index.js
@@ -20,6 +20,7 @@ import './style.css';
 import { Options } from '../../../components/options-context-provider';
 import { User } from '../user-context-provider';
 import { READER } from '../../../common/constants';
+import { ReaderThemes } from '../../../components/reader-themes-context-provider';
 
 /**
  * Navigation component.
@@ -36,6 +37,7 @@ export function Nav( { closeLink, finishLink } ) {
 		originalOptions: { preview_permalink: previewPermalink, reader_theme: readerTheme },
 	} = useContext( Options );
 	const { savingDeveloperToolsOption } = useContext( User );
+	const { downloadingTheme } = useContext( ReaderThemes );
 
 	let nextText;
 	let nextLink;
@@ -97,7 +99,7 @@ export function Nav( { closeLink, finishLink } ) {
 					}
 
 					<Button
-						disabled={ ! canGoForward || savingOptions || savingDeveloperToolsOption }
+						disabled={ ! canGoForward || savingOptions || savingDeveloperToolsOption || downloadingTheme }
 						href={ nextLink }
 						id="next-button"
 						isPrimary
diff --git a/assets/src/onboarding-wizard/components/nav/test/index.js b/assets/src/onboarding-wizard/components/nav/test/index.js
index 59decb17a53..bd648a0b747 100644
--- a/assets/src/onboarding-wizard/components/nav/test/index.js
+++ b/assets/src/onboarding-wizard/components/nav/test/index.js
@@ -17,9 +17,11 @@ import { Nav } from '..';
 import { NavigationContextProvider } from '../../navigation-context-provider';
 import { UserContextProvider } from '../../user-context-provider';
 import { OptionsContextProvider } from '../../../../components/options-context-provider';
+import { ReaderThemesContextProvider } from '../../../../components/reader-themes-context-provider';
 import { STANDARD, READER } from '../../../../common/constants';
 
 jest.mock( '../../../../components/options-context-provider' );
+jest.mock( '../../../../components/reader-themes-context-provider' );
 jest.mock( '../../user-context-provider' );
 
 let container;
@@ -35,11 +37,13 @@ const testPages = [
 	{ PageComponent: MyPageComponent, slug: 'slug-2', title: 'Page 1' },
 ];
 
-const Providers = ( { children, pages, themeSupport = READER } ) => (
+const Providers = ( { children, pages, themeSupport = READER, downloadingTheme = false } ) => (
 	<OptionsContextProvider themeSupport={ themeSupport }>
 		<UserContextProvider>
 			<NavigationContextProvider pages={ pages }>
-				{ children }
+				<ReaderThemesContextProvider downloadingTheme={ downloadingTheme }>
+					{ children }
+				</ReaderThemesContextProvider>
 			</NavigationContextProvider>
 		</UserContextProvider>
 	</OptionsContextProvider>
@@ -48,6 +52,7 @@ Providers.propTypes = {
 	children: PropTypes.any,
 	pages: PropTypes.array,
 	themeSupport: PropTypes.string,
+	downloadingTheme: PropTypes.bool,
 };
 
 describe( 'Nav', () => {
diff --git a/assets/src/onboarding-wizard/pages/summary/desktop-screenshot.js b/assets/src/onboarding-wizard/pages/summary/desktop-screenshot.js
index 3d157a09a57..cb332193425 100644
--- a/assets/src/onboarding-wizard/pages/summary/desktop-screenshot.js
+++ b/assets/src/onboarding-wizard/pages/summary/desktop-screenshot.js
@@ -12,6 +12,7 @@ import { __ } from '@wordpress/i18n';
  * Internal dependencies
  */
 import { Desktop } from '../../components/desktop';
+import DesktopIcon from '../../../components/svg/desktop-icon.svg';
 
 export function DesktopScreenshot( { screenshot, name, description, url } ) {
 	return (
@@ -19,14 +20,18 @@ export function DesktopScreenshot( { screenshot, name, description, url } ) {
 
 			<div className="grid grid-1-2 summary-screenshot">
 				<Desktop>
-					<img
-						src={ screenshot }
-						alt={ name }
-						loading="lazy"
-						decoding="async"
-						height="900"
-						width="1200"
-					/>
+					{
+						screenshot ? (
+							<img
+								src={ screenshot }
+								alt={ name }
+								loading="lazy"
+								decoding="async"
+								height="900"
+								width="1200"
+							/>
+						) : <DesktopIcon />
+					}
 				</Desktop>
 				<div>
 					<h3>
@@ -50,6 +55,6 @@ export function DesktopScreenshot( { screenshot, name, description, url } ) {
 DesktopScreenshot.propTypes = {
 	description: PropTypes.string.isRequired,
 	name: PropTypes.string.isRequired,
-	screenshot: PropTypes.string.isRequired,
+	screenshot: PropTypes.string,
 	url: PropTypes.string.isRequired,
 };
diff --git a/assets/src/onboarding-wizard/pages/summary/reader.js b/assets/src/onboarding-wizard/pages/summary/reader.js
index d3da27f63b2..864a3a70e77 100644
--- a/assets/src/onboarding-wizard/pages/summary/reader.js
+++ b/assets/src/onboarding-wizard/pages/summary/reader.js
@@ -20,6 +20,8 @@ import { IconMobile } from '../../../components/svg/icon-mobile';
 import { Options } from '../../../components/options-context-provider';
 import { RedirectToggle } from '../../../components/redirect-toggle';
 import { ReaderThemes } from '../../../components/reader-themes-context-provider';
+import DesktopIcon from '../../../components/svg/desktop-icon.svg';
+import MobileIcon from '../../../components/svg/mobile-icon.svg';
 import { SummaryHeader } from './summary-header';
 
 /**
@@ -57,14 +59,18 @@ export function Reader( { currentTheme } ) {
 						</AMPInfo>
 
 						<Desktop>
-							<img
-								src={ currentTheme.screenshot }
-								alt={ currentTheme.name }
-								loading="lazy"
-								decoding="async"
-								height="900"
-								width="1200"
-							/>
+							{
+								currentTheme.screenshot ? (
+									<img
+										src={ currentTheme.screenshot }
+										alt={ currentTheme.name }
+										loading="lazy"
+										decoding="async"
+										height="900"
+										width="1200"
+									/>
+								) : <DesktopIcon />
+							}
 
 						</Desktop>
 
@@ -89,14 +95,18 @@ export function Reader( { currentTheme } ) {
 						</AMPInfo>
 
 						<Phone>
-							<img
-								src={ readerThemeData.screenshot_url }
-								alt={ readerThemeData.name }
-								loading="lazy"
-								decoding="async"
-								height="2165"
-								width="1000"
-							/>
+							{
+								readerThemeData.screenshot_url ? (
+									<img
+										src={ readerThemeData.screenshot_url }
+										alt={ readerThemeData.name }
+										loading="lazy"
+										decoding="async"
+										height="2165"
+										width="1000"
+									/>
+								) : <MobileIcon style={ { width: '100%' } } />
+							}
 						</Phone>
 
 						<h3>
diff --git a/assets/src/onboarding-wizard/pages/summary/style.css b/assets/src/onboarding-wizard/pages/summary/style.css
index 7f51d16792f..61728fca0ab 100644
--- a/assets/src/onboarding-wizard/pages/summary/style.css
+++ b/assets/src/onboarding-wizard/pages/summary/style.css
@@ -26,6 +26,7 @@
 .reader-summary__screens img {
 	width: 100%;
 	height: auto;
+	margin: auto;
 }
 
 .reader-summary__screens .selectable {
diff --git a/assets/src/onboarding-wizard/style.css b/assets/src/onboarding-wizard/style.css
index b59d2fa0492..f76f47e6294 100644
--- a/assets/src/onboarding-wizard/style.css
+++ b/assets/src/onboarding-wizard/style.css
@@ -125,7 +125,7 @@ body {
 	position: relative;
 
 	@media screen and (min-width: 783px) {
-		padding: 15px 21px 8px 0;
+		padding: 15px 10px 8px 0;
 	}
 }
 
diff --git a/assets/src/settings-page/style.css b/assets/src/settings-page/style.css
index 913536e8334..d8e4bcc036f 100644
--- a/assets/src/settings-page/style.css
+++ b/assets/src/settings-page/style.css
@@ -59,10 +59,6 @@
 	}
 }
 
-.amp-settings .theme-card .phone-inner {
-	max-height: 298px;
-}
-
 /* Supported templates section. */
 .supported-templates {
 	margin-bottom: 3rem;
diff --git a/includes/amp-helper-functions.php b/includes/amp-helper-functions.php
index 171849705c3..27b104b7113 100644
--- a/includes/amp-helper-functions.php
+++ b/includes/amp-helper-functions.php
@@ -364,11 +364,16 @@ function amp_is_canonical() {
  * @return bool
  */
 function amp_is_legacy() {
-	return (
-		AMP_Theme_Support::READER_MODE_SLUG === AMP_Options_Manager::get_option( Option::THEME_SUPPORT )
-		&&
-		ReaderThemes::DEFAULT_READER_THEME === AMP_Options_Manager::get_option( Option::READER_THEME )
-	);
+	if ( AMP_Theme_Support::READER_MODE_SLUG !== AMP_Options_Manager::get_option( Option::THEME_SUPPORT ) ) {
+		return false;
+	}
+
+	$reader_theme = AMP_Options_Manager::get_option( Option::READER_THEME );
+	if ( ReaderThemes::DEFAULT_READER_THEME === $reader_theme ) {
+		return true;
+	}
+
+	return ! wp_get_theme( $reader_theme )->exists();
 }
 
 /**
diff --git a/includes/options/class-amp-options-manager.php b/includes/options/class-amp-options-manager.php
index a05d547d600..7dc15e349a9 100644
--- a/includes/options/class-amp-options-manager.php
+++ b/includes/options/class-amp-options-manager.php
@@ -42,6 +42,7 @@ class AMP_Options_Manager {
 	public static function init() {
 		add_action( 'admin_notices', [ __CLASS__, 'render_php_css_parser_conflict_notice' ] );
 		add_action( 'admin_notices', [ __CLASS__, 'insecure_connection_notice' ] );
+		add_action( 'admin_notices', [ __CLASS__, 'reader_theme_fallback_notice' ] );
 	}
 
 	/**
@@ -441,7 +442,8 @@ public static function update_options( $options ) {
 	 * @return void
 	 */
 	public static function render_php_css_parser_conflict_notice() {
-		if ( 'toplevel_page_' . self::OPTION_NAME !== get_current_screen()->id ) {
+		$current_screen = get_current_screen();
+		if ( ! ( $current_screen instanceof WP_Screen ) || 'toplevel_page_' . self::OPTION_NAME !== $current_screen->id ) {
 			return;
 		}
 
@@ -484,6 +486,8 @@ public static function render_php_css_parser_conflict_notice() {
 	 * @return void
 	 */
 	public static function insecure_connection_notice() {
+		$current_screen = get_current_screen();
+
 		// is_ssl() only tells us whether the admin backend uses HTTPS here, so we add a few more sanity checks.
 		$uses_ssl = (
 			is_ssl()
@@ -493,7 +497,7 @@ public static function insecure_connection_notice() {
 			( strpos( get_bloginfo( 'url' ), 'https' ) === 0 )
 		);
 
-		if ( ! $uses_ssl && 'toplevel_page_' . self::OPTION_NAME === get_current_screen()->id ) {
+		if ( ! $uses_ssl && $current_screen instanceof WP_Screen && 'toplevel_page_' . self::OPTION_NAME === $current_screen->id ) {
 			printf(
 				'<div class="notice notice-warning"><p>%s</p></div>',
 				wp_kses(
@@ -510,4 +514,44 @@ public static function insecure_connection_notice() {
 			);
 		}
 	}
+
+	/**
+	 * Outputs an admin notice if the AMP Legacy Reader theme is used as a fallback.
+	 */
+	public static function reader_theme_fallback_notice() {
+		$current_screen = get_current_screen();
+
+		if ( ! ( $current_screen instanceof WP_Screen ) || ! in_array( $current_screen->id, [ 'themes', 'toplevel_page_' . self::OPTION_NAME ], true ) ) {
+			return;
+		}
+
+		$reader_themes = new ReaderThemes();
+
+		if ( $reader_themes->using_fallback_theme() && current_user_can( 'manage_options' ) ) {
+			$selected_theme = self::get_option( Option::READER_THEME );
+			$error_message  = sprintf(
+				/* translators: 1: slug of the Reader theme, 2: the URL for the reader theme selection UI */
+				__( 'The AMP Reader theme %1$s cannot be found. Your site is currently falling back to using the Legacy templates for AMP pages. Please <a href="%2$s">re-select</a> the desired Reader theme.', 'amp' ),
+				"<code>{$selected_theme}</code>",
+				esc_url( add_query_arg( 'page', self::OPTION_NAME, admin_url( 'admin.php' ) ) . '#reader-themes-drawer' )
+			);
+			?>
+			<div class="notice notice-warning">
+				<p>
+					<?php
+					echo wp_kses(
+						$error_message,
+						[
+							'code' => [],
+							'a'    => [
+								'href' => true,
+							],
+						]
+					);
+					?>
+				</p>
+			</div>
+			<?php
+		}
+	}
 }
diff --git a/package-lock.json b/package-lock.json
index fb1bfed571d..667526c35b3 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -4386,6 +4386,12 @@
           "integrity": "sha512-sGkPx+VjMtmA6MX27oA4FBFELFCZZ4S4XqeGOXCv68tT+jb3vk/RyaKWP0PTKyWtmLSM0b+adUTEvbs1PEaH2w==",
           "dev": true
         },
+        "prettier": {
+          "version": "npm:wp-prettier@2.0.5",
+          "resolved": "https://registry.npmjs.org/wp-prettier/-/wp-prettier-2.0.5.tgz",
+          "integrity": "sha512-5GCgdeevIXwR3cW4Qj5XWC5MO1iSCz8+IPn0mMw6awAt/PBiey8yyO7MhePRsaMqghJAhg6Q3QLYWSnUHWkG6A==",
+          "dev": true
+        },
         "semver": {
           "version": "6.3.0",
           "resolved": "https://registry.npmjs.org/semver/-/semver-6.3.0.tgz",
@@ -19092,12 +19098,6 @@
       "integrity": "sha1-1PRWKwzjaW5BrFLQ4ALlemNdxtw=",
       "dev": true
     },
-    "prettier": {
-      "version": "npm:wp-prettier@2.0.5",
-      "resolved": "https://registry.npmjs.org/wp-prettier/-/wp-prettier-2.0.5.tgz",
-      "integrity": "sha512-5GCgdeevIXwR3cW4Qj5XWC5MO1iSCz8+IPn0mMw6awAt/PBiey8yyO7MhePRsaMqghJAhg6Q3QLYWSnUHWkG6A==",
-      "dev": true
-    },
     "prettier-linter-helpers": {
       "version": "1.0.0",
       "resolved": "https://registry.npmjs.org/prettier-linter-helpers/-/prettier-linter-helpers-1.0.0.tgz",
diff --git a/src/Admin/OnboardingWizardSubmenuPage.php b/src/Admin/OnboardingWizardSubmenuPage.php
index f32d81822af..0fe6f37e60f 100644
--- a/src/Admin/OnboardingWizardSubmenuPage.php
+++ b/src/Admin/OnboardingWizardSubmenuPage.php
@@ -14,6 +14,7 @@
 use AmpProject\AmpWP\Infrastructure\Delayed;
 use AmpProject\AmpWP\Infrastructure\Registerable;
 use AmpProject\AmpWP\Infrastructure\Service;
+use AmpProject\AmpWP\Option;
 use AmpProject\AmpWP\QueryVar;
 use AmpProject\AmpWP\Services;
 
@@ -212,7 +213,7 @@ public function enqueue_assets( $hook_suffix ) {
 		wp_styles()->add_data( self::ASSET_HANDLE, 'rtl', 'replace' );
 
 		$theme           = wp_get_theme();
-		$is_reader_theme = in_array( get_stylesheet(), wp_list_pluck( $this->reader_themes->get_themes(), 'slug' ), true );
+		$is_reader_theme = $this->reader_themes->theme_data_exists( get_stylesheet() );
 
 		$exit_link = menu_page_url( AMP_Options_Manager::OPTION_NAME, false );
 
@@ -235,9 +236,10 @@ public function enqueue_assets( $hook_suffix ) {
 				'name'            => $theme->get( 'Name' ),
 				'description'     => $theme->get( 'Description' ),
 				'is_reader_theme' => $is_reader_theme,
-				'screenshot'      => $theme->get_screenshot(),
+				'screenshot'      => $theme->get_screenshot() ?: null,
 				'url'             => $theme->get( 'ThemeURI' ),
 			],
+			'USING_FALLBACK_READER_THEME'        => $this->reader_themes->using_fallback_theme(),
 			'FINISH_LINK'                        => $exit_link,
 			'OPTIONS_REST_PATH'                  => '/amp/v1/options',
 			'READER_THEMES_REST_PATH'            => '/amp/v1/reader-themes',
diff --git a/src/Admin/OptionsMenu.php b/src/Admin/OptionsMenu.php
index ea4e033cb06..320e9d83ac3 100644
--- a/src/Admin/OptionsMenu.php
+++ b/src/Admin/OptionsMenu.php
@@ -13,6 +13,7 @@
 use AmpProject\AmpWP\Infrastructure\Conditional;
 use AmpProject\AmpWP\Infrastructure\Registerable;
 use AmpProject\AmpWP\Infrastructure\Service;
+use AmpProject\AmpWP\Option;
 
 /**
  * OptionsMenu class.
@@ -193,26 +194,28 @@ public function enqueue_assets( $hook_suffix ) {
 		wp_styles()->add_data( self::ASSET_HANDLE, 'rtl', 'replace' );
 
 		$theme           = wp_get_theme();
-		$is_reader_theme = in_array( get_stylesheet(), wp_list_pluck( $this->reader_themes->get_themes(), 'slug' ), true );
+		$is_reader_theme = $this->reader_themes->theme_data_exists( get_stylesheet() );
 
 		$js_data = [
-			'CURRENT_THEME'              => [
+			'CURRENT_THEME'               => [
 				'name'            => $theme->get( 'Name' ),
 				'description'     => $theme->get( 'Description' ),
 				'is_reader_theme' => $is_reader_theme,
-				'screenshot'      => $theme->get_screenshot(),
+				'screenshot'      => $theme->get_screenshot() ?: null,
 				'url'             => $theme->get( 'ThemeURI' ),
 			],
-			'OPTIONS_REST_PATH'          => '/amp/v1/options',
-			'READER_THEMES_REST_PATH'    => '/amp/v1/reader-themes',
-			'IS_CORE_THEME'              => in_array(
+			'OPTIONS_REST_PATH'           => '/amp/v1/options',
+			'READER_THEMES_REST_PATH'     => '/amp/v1/reader-themes',
+			'IS_CORE_THEME'               => in_array(
 				get_stylesheet(),
 				AMP_Core_Theme_Sanitizer::get_supported_themes(),
 				true
 			),
-			'THEME_SUPPORT_ARGS'         => AMP_Theme_Support::get_theme_support_args(),
-			'THEME_SUPPORTS_READER_MODE' => AMP_Theme_Support::supports_reader_mode(),
-			'UPDATES_NONCE'              => wp_create_nonce( 'updates' ),
+			'LEGACY_THEME_SLUG'           => ReaderThemes::DEFAULT_READER_THEME,
+			'USING_FALLBACK_READER_THEME' => $this->reader_themes->using_fallback_theme(),
+			'THEME_SUPPORT_ARGS'          => AMP_Theme_Support::get_theme_support_args(),
+			'THEME_SUPPORTS_READER_MODE'  => AMP_Theme_Support::supports_reader_mode(),
+			'UPDATES_NONCE'               => wp_create_nonce( 'updates' ),
 		];
 
 		wp_add_inline_script(
diff --git a/src/Admin/ReaderThemes.php b/src/Admin/ReaderThemes.php
index e720330831c..9f2a674bd6a 100644
--- a/src/Admin/ReaderThemes.php
+++ b/src/Admin/ReaderThemes.php
@@ -9,6 +9,9 @@
 namespace AmpProject\AmpWP\Admin;
 
 use AMP_Core_Theme_Sanitizer;
+use AMP_Options_Manager;
+use AmpProject\AmpWP\Option;
+use WP_Theme;
 use WP_Upgrader;
 
 /**
@@ -105,10 +108,32 @@ public function get_themes() {
 		 */
 		$themes = (array) apply_filters( 'amp_reader_themes', $themes );
 
+		$selected_theme_slug = AMP_Options_Manager::get_option( Option::READER_THEME );
+		$theme_slugs         = wp_list_pluck( $themes, 'slug' );
+
+		/*
+		 * Check if the chosen Reader theme is among the list of filtered themes. If not, an attempt will be made to
+		 * obtain the theme data from the list of installed themes. If neither case is true, the AMP Legacy theme will
+		 * be used as a fallback.
+		 */
+		if ( self::DEFAULT_READER_THEME !== $selected_theme_slug && ! in_array( $selected_theme_slug, $theme_slugs, true ) ) {
+			$active_theme = wp_get_theme( $selected_theme_slug );
+
+			if ( $active_theme->exists() ) {
+				$themes[] = $this->normalize_theme_data( $active_theme );
+			}
+		}
+
+		/*
+		 * Append the AMP Legacy theme details after filtering the default themes. This ensures the AMP Legacy theme
+		 * will always be available as a fallback if the chosen Reader theme becomes unavailable.
+		 */
+		$themes[] = $this->get_legacy_theme();
+
 		$themes = array_filter(
 			$themes,
 			static function( $theme ) {
-				return is_array( $theme ) && ! empty( $theme ) && ! empty( $theme['screenshot_url'] ); // Screenshots are required.
+				return is_array( $theme ) && ! empty( $theme );
 			}
 		);
 
@@ -172,7 +197,7 @@ public function get_default_reader_themes() {
 		}
 
 		if ( is_wp_error( $response ) ) {
-			return [ $this->get_legacy_theme() ];
+			return [];
 		}
 
 		if ( is_array( $response ) ) {
@@ -192,6 +217,48 @@ static function ( $theme ) use ( $supported_themes ) {
 			}
 		);
 
+		$reader_themes = array_map(
+			function ( $theme ) {
+				$theme_data                   = $this->normalize_theme_data( $theme );
+				$theme_data['screenshot_url'] = amp_get_asset_url( "images/reader-themes/{$theme_data['slug']}.jpg" );
+
+				return $theme_data;
+			},
+			$reader_themes
+		);
+
+		$this->default_reader_themes = $reader_themes;
+		return $this->default_reader_themes;
+	}
+
+	/**
+	 * Normalize the specified theme data.
+	 *
+	 * @param WP_Theme|array|\stdClass $theme Theme.
+	 * @return array Normalized theme data.
+	 */
+	private function normalize_theme_data( $theme ) {
+		if ( $theme instanceof WP_Theme ) {
+			if ( $theme->errors() ) {
+				return [];
+			}
+
+			return [
+				'name'           => $theme->display( 'Name' ),
+				'slug'           => $theme->get_stylesheet(),
+				'preview_url'    => null,
+				'screenshot_url' => $theme->get_screenshot(),
+				'homepage'       => $theme->display( 'ThemeURI' ),
+				'description'    => $theme->display( 'Description' ),
+				'requires'       => $theme->get( 'RequiresWP' ),
+				'requires_php'   => $theme->get( 'RequiresPHP' ),
+			];
+		}
+
+		if ( ! is_array( $theme ) && ! is_object( $theme ) ) {
+			return [];
+		}
+
 		$keys = [
 			'name',
 			'slug',
@@ -203,22 +270,10 @@ static function ( $theme ) use ( $supported_themes ) {
 			'requires_php',
 		];
 
-		// Supply the screenshots.
-		$reader_themes = array_map(
-			static function ( $theme ) use ( $keys ) {
-				return array_merge(
-					array_fill_keys( $keys, '' ), // Provide empty defaults to make sure all keys are present.
-					wp_array_slice_assoc( (array) $theme, $keys ),
-					[ 'screenshot_url' => amp_get_asset_url( "images/reader-themes/{$theme->slug}.jpg" ) ]
-				);
-			},
-			$reader_themes
+		return array_merge(
+			array_fill_keys( $keys, '' ),
+			wp_array_slice_assoc( (array) $theme, $keys )
 		);
-
-		$reader_themes[] = $this->get_legacy_theme();
-
-		$this->default_reader_themes = $reader_themes;
-		return $this->default_reader_themes;
 	}
 
 	/**
@@ -281,6 +336,26 @@ public function get_theme_availability( $theme ) {
 		}
 	}
 
+	/**
+	 * Determine if the data for the specified Reader theme exists.
+	 *
+	 * @param string $theme_slug Theme slug.
+	 * @return bool Whether the Reader theme data exists.
+	 */
+	public function theme_data_exists( $theme_slug ) {
+		return in_array( $theme_slug, wp_list_pluck( $this->get_themes(), 'slug' ), true );
+	}
+
+	/**
+	 * Determine if the AMP legacy Reader theme is being used as a fallback.
+	 *
+	 * @return bool True if being used as a fallback, false otherwise.
+	 */
+	public function using_fallback_theme() {
+		return amp_is_legacy()
+			&& self::DEFAULT_READER_THEME !== AMP_Options_Manager::get_option( Option::READER_THEME );
+	}
+
 	/**
 	 * Provides details for the legacy theme included with the plugin.
 	 *
diff --git a/src/Infrastructure/ServiceBasedPlugin.php b/src/Infrastructure/ServiceBasedPlugin.php
index bb302366b04..488b6d7dd46 100644
--- a/src/Infrastructure/ServiceBasedPlugin.php
+++ b/src/Infrastructure/ServiceBasedPlugin.php
@@ -9,7 +9,6 @@
 
 use AmpProject\AmpWP\Exception\InvalidService;
 use AmpProject\AmpWP\Infrastructure\ServiceContainer\LazilyInstantiatedService;
-use AmpProject\AmpWP\PluginRegistry;
 
 /**
  * This abstract base plugin provides all the boilerplate code for working with
diff --git a/src/OptionsRESTController.php b/src/OptionsRESTController.php
index ef2398083e2..91f6f8c870d 100644
--- a/src/OptionsRESTController.php
+++ b/src/OptionsRESTController.php
@@ -277,7 +277,7 @@ public function get_item_schema() {
 						'arg_options' => [
 							'validate_callback' => function ( $value ) {
 								// Note: The validate_callback is used instead of enum in order to prevent leaking the list of themes.
-								return in_array( $value, wp_list_pluck( $this->reader_themes->get_themes(), 'slug' ), true );
+								return $this->reader_themes->theme_data_exists( $value );
 							},
 						],
 					],
diff --git a/src/ReaderThemeLoader.php b/src/ReaderThemeLoader.php
index 8b642349dba..006481667a5 100644
--- a/src/ReaderThemeLoader.php
+++ b/src/ReaderThemeLoader.php
@@ -67,7 +67,13 @@ public function is_enabled() {
 
 		// If the Legacy Reader mode is active, then a Reader theme is not going to be served.
 		$reader_theme = AMP_Options_Manager::get_option( Option::READER_THEME );
-		if ( ReaderThemes::DEFAULT_READER_THEME === AMP_Options_Manager::get_option( Option::READER_THEME ) ) {
+		if ( ReaderThemes::DEFAULT_READER_THEME === $reader_theme ) {
+			return false;
+		}
+
+		// If the Reader theme does not exist, then we cannot switch to the reader theme. The Legacy Reader theme will
+		// be used as a fallback instead.
+		if ( ! wp_get_theme( $reader_theme )->exists() ) {
 			return false;
 		}
 
diff --git a/templates/style.php b/templates/style.php
index 1a4ac099f20..31e9311e4c4 100644
--- a/templates/style.php
+++ b/templates/style.php
@@ -161,7 +161,8 @@
 .amp-wp-tax-tag,
 .amp-wp-comments-link,
 .amp-wp-footer p,
-.back-to-top {
+.back-to-top,
+#amp-mobile-version-switcher {
 	font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen-Sans", "Ubuntu", "Cantarell", "Helvetica Neue", sans-serif;
 }
 
diff --git a/tests/e2e/specs/amp-onboarding/template-mode.js b/tests/e2e/specs/amp-onboarding/template-mode.js
index 2f9fc41709f..65414f13b06 100644
--- a/tests/e2e/specs/amp-onboarding/template-mode.js
+++ b/tests/e2e/specs/amp-onboarding/template-mode.js
@@ -88,12 +88,12 @@ describe( 'Stepper item modifications', () => {
 describe( 'Template mode recommendations with non-reader-theme active', () => {
 	beforeEach( async () => {
 		await cleanUpSettings();
-		await installTheme( 'astra' );
-		await activateTheme( 'astra' );
+		await installTheme( 'neve' );
+		await activateTheme( 'neve' );
 	} );
 
 	afterEach( async () => {
-		await deleteTheme( 'astra', 'twentytwenty' );
+		await deleteTheme( 'neve', 'twentytwenty' );
 	} );
 
 	it( 'makes correct recommendations when user is not technical and the current theme is not a reader theme', async () => {
diff --git a/tests/js/jest.config.js b/tests/js/jest.config.js
index 18d4cdc5738..1c59c31c26e 100644
--- a/tests/js/jest.config.js
+++ b/tests/js/jest.config.js
@@ -18,6 +18,9 @@ module.exports = {
 		'<rootDir>/build/',
 		'<rootDir>/tests/shared',
 	],
+	modulePathIgnorePatterns: [
+		'<rootDir>/assets/src/components/.*/__mocks__',
+	],
 	coverageReporters: [ 'lcov' ],
 	coverageDirectory: '<rootDir>/build/logs',
 	reporters: [ [ 'jest-silent-reporter', { useDots: true } ] ],
diff --git a/tests/php/data/themes/child-of-core/style.css b/tests/php/data/themes/child-of-core/style.css
index f0729255df4..6c36fea90ef 100644
--- a/tests/php/data/themes/child-of-core/style.css
+++ b/tests/php/data/themes/child-of-core/style.css
@@ -1,4 +1,4 @@
 /**
-Theme Name: Child o' Twenty Twenty
-Template: twentytwenty
+Theme Name: Child o' Twenty Seventeen
+Template: twentyseventeen
  */
diff --git a/tests/php/src/Admin/ReaderThemesTest.php b/tests/php/src/Admin/ReaderThemesTest.php
index b64facdd0a0..d7fab20f8ab 100644
--- a/tests/php/src/Admin/ReaderThemesTest.php
+++ b/tests/php/src/Admin/ReaderThemesTest.php
@@ -8,7 +8,11 @@
 
 namespace AmpProject\AmpWP\Tests\Admin;
 
+use AMP_Options_Manager;
+use AMP_Theme_Support;
 use AmpProject\AmpWP\Admin\ReaderThemes;
+use AmpProject\AmpWP\Option;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\AmpWP\Tests\Helpers\ThemesApiRequestMocking;
 use WP_UnitTestCase;
 use Closure;
@@ -22,7 +26,7 @@
  */
 class ReaderThemesTest extends WP_UnitTestCase {
 
-	use ThemesApiRequestMocking;
+	use ThemesApiRequestMocking, LoadsCoreThemes;
 
 	/**
 	 * Test instance.
@@ -31,8 +35,6 @@ class ReaderThemesTest extends WP_UnitTestCase {
 	 */
 	private $reader_themes;
 
-	private $original_theme_directories;
-
 	/**
 	 * Setup.
 	 *
@@ -50,17 +52,13 @@ public function setUp() {
 		switch_theme( 'twentytwenty' );
 		$this->reader_themes = new ReaderThemes();
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	public function tearDown() {
 		parent::tearDown();
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+
+		$this->restore_theme_directories();
 	}
 
 	/**
@@ -89,6 +87,16 @@ public function test_get_themes() {
 		foreach ( $themes as $theme ) {
 			$this->assertEqualSets( $keys, array_keys( $theme ) );
 		}
+
+		// Verify that the Reader theme data can be retrieved from the list of installed themes.
+		register_theme_directory( __DIR__ . '/../../data/themes' );
+		delete_site_transient( 'theme_roots' );
+
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'child-of-core' );
+
+		$themes = ( new ReaderThemes() )->get_themes();
+
+		$this->assertContains( 'child-of-core', wp_list_pluck( $themes, 'slug' ) );
 	}
 
 	/**
@@ -237,4 +245,48 @@ public function test_can_install_theme() {
 		$core_theme['requires_php'] = '999.9';
 		$this->assertFalse( $this->reader_themes->can_install_theme( $core_theme ) );
 	}
+
+	/**
+	 * Tests for theme_data_exists.
+	 *
+	 * @covers ReaderThemes::theme_data_exists
+	 */
+	public function test_theme_data_exists() {
+		$this->assertFalse( ( new ReaderThemes() )->theme_data_exists( 'neve' ) );
+
+		$neve_theme        = [
+			'name'         => 'Neve',
+			'requires'     => false,
+			'requires_php' => '5.2',
+			'slug'         => 'neve',
+		];
+		$append_neve_theme = static function ( $themes ) use ( $neve_theme ) {
+			$themes[] = $neve_theme;
+			return $themes;
+		};
+
+		add_filter( 'amp_reader_themes', $append_neve_theme );
+
+		$this->assertTrue( ( new ReaderThemes() )->theme_data_exists( 'neve' ) );
+
+		remove_filter( 'amp_reader_themes', $append_neve_theme );
+	}
+
+	/** @covers ReaderThemes::using_fallback_theme */
+	public function test_using_fallback_theme() {
+		$reader_themes = new ReaderThemes();
+		AMP_Options_Manager::update_options(
+			[
+				Option::THEME_SUPPORT => 'reader',
+				Option::READER_THEME  => ReaderThemes::DEFAULT_READER_THEME,
+			]
+		);
+		$this->assertFalse( $reader_themes->using_fallback_theme() );
+
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'foobar' );
+		$this->assertTrue( $reader_themes->using_fallback_theme() );
+
+		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::TRANSITIONAL_MODE_SLUG );
+		$this->assertFalse( $reader_themes->using_fallback_theme() );
+	}
 }
diff --git a/tests/php/src/Helpers/LoadsCoreThemes.php b/tests/php/src/Helpers/LoadsCoreThemes.php
new file mode 100644
index 00000000000..aee26a6da9e
--- /dev/null
+++ b/tests/php/src/Helpers/LoadsCoreThemes.php
@@ -0,0 +1,39 @@
+<?php
+/**
+ * Trait LoadsCoreThemes.
+ *
+ * @package AmpProject\AmpWP
+ */
+
+namespace AmpProject\AmpWP\Tests\Helpers;
+
+/**
+ * Helper trait for loading core themes.
+ *
+ * @package AmpProject\AmpWP
+ */
+trait LoadsCoreThemes {
+
+	private $original_theme_directories;
+
+	/**
+	 * Register the core theme directory.
+	 */
+	private function register_core_themes() {
+		global $wp_theme_directories;
+
+		$this->original_theme_directories = $wp_theme_directories;
+		register_theme_directory( ABSPATH . 'wp-content/themes' );
+		delete_site_transient( 'theme_roots' );
+	}
+
+	/**
+	 * Restore the original theme directories.
+	 */
+	private function restore_theme_directories() {
+		global $wp_theme_directories;
+
+		$wp_theme_directories = $this->original_theme_directories;
+		delete_site_transient( 'theme_roots' );
+	}
+}
diff --git a/tests/php/src/ReaderThemeLoaderTest.php b/tests/php/src/ReaderThemeLoaderTest.php
index 27795d0e7e6..6a53a4e1d50 100644
--- a/tests/php/src/ReaderThemeLoaderTest.php
+++ b/tests/php/src/ReaderThemeLoaderTest.php
@@ -11,6 +11,7 @@
 use AmpProject\AmpWP\Option;
 use AmpProject\AmpWP\ReaderThemeLoader;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use WP_Customize_Manager;
 use WP_Customize_Panel;
 use WP_Theme;
@@ -19,28 +20,22 @@
 /** @covers ReaderThemeLoader */
 final class ReaderThemeLoaderTest extends WP_UnitTestCase {
 
-	use AssertContainsCompatibility;
+	use AssertContainsCompatibility, LoadsCoreThemes;
 
 	/** @var ReaderThemeLoader */
 	private $instance;
 
-	private $original_theme_directories;
-
 	public function setUp() {
 		parent::setUp();
 		$this->instance = new ReaderThemeLoader();
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	public function tearDown() {
 		parent::tearDown();
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+
+		$this->restore_theme_directories();
 	}
 
 	/** @covers ReaderThemeLoader::is_enabled() */
diff --git a/tests/php/test-amp-gallery-embed-handler.php b/tests/php/test-amp-gallery-embed-handler.php
index fb639cf4fbc..7ba72412f9b 100644
--- a/tests/php/test-amp-gallery-embed-handler.php
+++ b/tests/php/test-amp-gallery-embed-handler.php
@@ -7,6 +7,7 @@
 
 use AmpProject\AmpWP\Admin\ReaderThemes;
 use AmpProject\AmpWP\Option;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\AmpWP\Tests\Helpers\WithoutBlockPreRendering;
 
 /**
@@ -14,7 +15,7 @@
  */
 class AMP_Gallery_Embed_Handler_Test extends WP_UnitTestCase {
 
-	use WithoutBlockPreRendering;
+	use WithoutBlockPreRendering, LoadsCoreThemes;
 
 	private static $original_amp_options;
 
@@ -26,14 +27,23 @@ public static function tearDownAfterClass() {
 		AMP_Options_Manager::update_options( self::$original_amp_options );
 	}
 
+	public function setUp() {
+		parent::setUp();
+
+		$this->register_core_themes();
+	}
+
 	/**
 	 * Tear down.
 	 */
 	public function tearDown() {
+		parent::tearDown();
+
 		if ( did_action( 'add_attachment' ) ) {
 			$this->remove_added_uploads();
 		}
-		parent::tearDown();
+
+		$this->restore_theme_directories();
 	}
 
 	/**
@@ -181,7 +191,7 @@ public function test__conversion( $source, $expected, $use_legacy_mode = false )
 			AMP_Options_Manager::update_option( Option::READER_THEME, ReaderThemes::DEFAULT_READER_THEME );
 			AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
 		} else {
-			AMP_Options_Manager::update_option( Option::READER_THEME, 'foobar' );
+			AMP_Options_Manager::update_option( Option::READER_THEME, 'twentyseventeen' );
 		}
 
 		// When we generate new attachments, we neither control the IDs being
diff --git a/tests/php/test-amp-helper-functions.php b/tests/php/test-amp-helper-functions.php
index 379c4fdecd7..a846e016173 100644
--- a/tests/php/test-amp-helper-functions.php
+++ b/tests/php/test-amp-helper-functions.php
@@ -9,6 +9,7 @@
 use AmpProject\AmpWP\QueryVar;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
 use AmpProject\AmpWP\Tests\Helpers\HandleValidation;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 
 /**
  * Class Test_AMP_Helper_Functions
@@ -17,6 +18,7 @@ class Test_AMP_Helper_Functions extends WP_UnitTestCase {
 
 	use AssertContainsCompatibility;
 	use HandleValidation;
+	use LoadsCoreThemes;
 
 	/**
 	 * The mock Site Icon value to use in a filter.
@@ -39,6 +41,8 @@ public function setUp() {
 		parent::setUp();
 		$this->server_var_backup = $_SERVER;
 		remove_theme_support( 'amp' );
+
+		$this->register_core_themes();
 	}
 
 	/**
@@ -60,6 +64,8 @@ public function tearDown() {
 		$wp_rewrite->init();
 		$wp_rewrite->flush_rules();
 
+		$this->restore_theme_directories();
+
 		if ( class_exists( 'WP_Block_Type_Registry' ) ) {
 			foreach ( WP_Block_Type_Registry::get_instance()->get_all_registered() as $block ) {
 				if ( 'amp/' === substr( $block->name, 0, 4 ) ) {
@@ -278,8 +284,12 @@ public function test_amp_is_legacy() {
 		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
 		$this->assertTrue( amp_is_legacy() );
 
-		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentynineteen' );
+		$this->assertTrue( wp_get_theme( 'twentyseventeen' )->exists() );
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentyseventeen' );
 		$this->assertFalse( amp_is_legacy() );
+
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'foobar' );
+		$this->assertTrue( amp_is_legacy() );
 	}
 
 	/**
@@ -1746,22 +1756,25 @@ public function test_amp_add_admin_bar_item() {
 		$this->go_to( amp_get_permalink( $post_id ) );
 
 		// Confirm legacy Reader mode works.
-		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
-		$this->assertTrue( amp_is_legacy() );
-		$admin_bar = new WP_Admin_Bar();
-		wp_admin_bar_customize_menu( $admin_bar );
-		amp_add_admin_bar_view_link( $admin_bar );
-		$item = $admin_bar->get_node( 'amp' );
-		$this->assertInternalType( 'object', $item );
-		$this->assertEquals( esc_url( get_permalink( $post_id ) ), $item->href );
-		$item = $admin_bar->get_node( 'customize' );
-		$this->assertInternalType( 'object', $item );
-		$this->assertStringNotContains( amp_get_slug() . '=', $item->href );
-		$this->assertStringContains( 'autofocus', $item->href );
+		foreach ( [ AMP_Theme_Support::READER_MODE_SLUG, 'foobar' ] as $reader_theme ) {
+			AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
+			AMP_Options_Manager::update_option( Option::READER_THEME, $reader_theme );
+			$this->assertTrue( amp_is_legacy() );
+			$admin_bar = new WP_Admin_Bar();
+			wp_admin_bar_customize_menu( $admin_bar );
+			amp_add_admin_bar_view_link( $admin_bar );
+			$item = $admin_bar->get_node( 'amp' );
+			$this->assertInternalType( 'object', $item );
+			$this->assertEquals( esc_url( get_permalink( $post_id ) ), $item->href );
+			$item = $admin_bar->get_node( 'customize' );
+			$this->assertInternalType( 'object', $item );
+			$this->assertStringNotContains( amp_get_slug() . '=', $item->href );
+			$this->assertStringContains( 'autofocus', $item->href );
+		}
 
 		// Confirm Customize link with a Reader theme points to the right place.
 		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
-		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentynineteen' );
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentyseventeen' );
 		$this->assertFalse( amp_is_legacy() );
 		$admin_bar = new WP_Admin_Bar();
 		wp_admin_bar_customize_menu( $admin_bar );
diff --git a/tests/php/test-amp-style-sanitizer.php b/tests/php/test-amp-style-sanitizer.php
index bd3a0b7328f..5dd6543e1be 100644
--- a/tests/php/test-amp-style-sanitizer.php
+++ b/tests/php/test-amp-style-sanitizer.php
@@ -11,6 +11,7 @@
 use AmpProject\AmpWP\RemoteRequest\CachedResponse;
 use AmpProject\AmpWP\RemoteRequest\CachedRemoteGetRequest;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\AmpWP\Tests\Helpers\MarkupComparison;
 use AmpProject\Dom\Document;
 use AmpProject\AmpWP\Tests\Helpers\PrivateAccess;
@@ -24,8 +25,7 @@ class AMP_Style_Sanitizer_Test extends WP_UnitTestCase {
 	use AssertContainsCompatibility;
 	use MarkupComparison;
 	use PrivateAccess;
-
-	private $original_theme_directories;
+	use LoadsCoreThemes;
 
 	/**
 	 * Set up.
@@ -37,10 +37,7 @@ public function setUp() {
 		$wp_scripts = null;
 		delete_option( AMP_Options_Manager::OPTION_NAME ); // Make sure default reader mode option does not override theme support being added.
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	/**
@@ -53,9 +50,7 @@ public function tearDown() {
 		$wp_scripts   = null;
 		$wp_customize = null;
 
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+		$this->restore_theme_directories();
 	}
 
 	/**
diff --git a/tests/php/test-class-amp-core-theme-sanitizer.php b/tests/php/test-class-amp-core-theme-sanitizer.php
index a32ce060eaf..faf2574bde7 100644
--- a/tests/php/test-class-amp-core-theme-sanitizer.php
+++ b/tests/php/test-class-amp-core-theme-sanitizer.php
@@ -6,6 +6,7 @@
  */
 
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\Dom\Document;
 use AmpProject\AmpWP\Tests\Helpers\PrivateAccess;
 
@@ -16,23 +17,18 @@ class AMP_Core_Theme_Sanitizer_Test extends WP_UnitTestCase {
 
 	use AssertContainsCompatibility;
 	use PrivateAccess;
-
-	private $original_theme_directories;
+	use LoadsCoreThemes;
 
 	public function setUp() {
 		parent::setUp();
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	public function tearDown() {
 		parent::tearDown();
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+
+		$this->restore_theme_directories();
 	}
 
 	/**
@@ -193,9 +189,9 @@ public function test_extend_theme_support() {
 		switch_theme( 'child-of-core' );
 		AMP_Core_Theme_Sanitizer::extend_theme_support();
 		$this->assertTrue( current_theme_supports( 'amp' ) );
-		$this->assertEquals(
-			[ 'paired' => true ],
-			AMP_Theme_Support::get_theme_support_args()
+		$this->assertEqualSets(
+			[ 'paired', 'nav_menu_toggle', 'nav_menu_dropdown' ],
+			array_keys( AMP_Theme_Support::get_theme_support_args() )
 		);
 	}
 
diff --git a/tests/php/test-class-amp-options-manager.php b/tests/php/test-class-amp-options-manager.php
index e61c37fd5bd..2e3d3e69336 100644
--- a/tests/php/test-class-amp-options-manager.php
+++ b/tests/php/test-class-amp-options-manager.php
@@ -7,6 +7,8 @@
 
 use AmpProject\AmpWP\Option;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
+use AmpProject\AmpWP\Admin\ReaderThemes;
 
 /**
  * Tests for AMP_Options_Manager.
@@ -15,7 +17,7 @@
  */
 class Test_AMP_Options_Manager extends WP_UnitTestCase {
 
-	use AssertContainsCompatibility;
+	use AssertContainsCompatibility, LoadsCoreThemes;
 
 	/**
 	 * Whether the external object cache was enabled.
@@ -24,8 +26,6 @@ class Test_AMP_Options_Manager extends WP_UnitTestCase {
 	 */
 	private $was_wp_using_ext_object_cache;
 
-	private $original_theme_directories;
-
 	/**
 	 * Set up.
 	 */
@@ -36,10 +36,7 @@ public function setUp() {
 		remove_theme_support( 'amp' );
 		$GLOBALS['wp_settings_errors'] = [];
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	/**
@@ -51,13 +48,14 @@ public function tearDown() {
 		unregister_post_type( 'foo' );
 		unregister_post_type( 'book' );
 
+		global $current_screen;
+		$current_screen = null;
+
 		foreach ( get_post_types() as $post_type ) {
 			remove_post_type_support( $post_type, 'amp' );
 		}
 
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+		$this->restore_theme_directories();
 	}
 
 	/**
@@ -76,6 +74,7 @@ public function test_init() {
 		AMP_Options_Manager::init();
 		$this->assertEquals( 10, has_action( 'admin_notices', [ AMP_Options_Manager::class, 'render_php_css_parser_conflict_notice' ] ) );
 		$this->assertEquals( 10, has_action( 'admin_notices', [ AMP_Options_Manager::class, 'insecure_connection_notice' ] ) );
+		$this->assertEquals( 10, has_action( 'admin_notices', [ AMP_Options_Manager::class, 'reader_theme_fallback_notice' ] ) );
 	}
 
 	/**
@@ -657,4 +656,66 @@ public function test_get_options_theme_support_defaults( $args, $expected_mode,
 		}
 		$this->assertEquals( $expected_mode, AMP_Options_Manager::get_option( Option::THEME_SUPPORT ) );
 	}
+
+	/** @covers AMP_Options_Manager::render_php_css_parser_conflict_notice() */
+	public function test_render_php_css_parser_conflict_notice() {
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'render_php_css_parser_conflict_notice' ] ) );
+
+		set_current_screen( 'themes' );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'render_php_css_parser_conflict_notice' ] ) );
+
+		set_current_screen( 'toplevel_page_' . AMP_Options_Manager::OPTION_NAME );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'render_php_css_parser_conflict_notice' ] ) );
+	}
+
+	/** @covers AMP_Options_Manager::insecure_connection_notice() */
+	public function test_insecure_connection_notice() {
+		$_SERVER['HTTPS'] = false;
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'insecure_connection_notice' ] ) );
+
+		set_current_screen( 'themes' );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'insecure_connection_notice' ] ) );
+
+		set_current_screen( 'toplevel_page_' . AMP_Options_Manager::OPTION_NAME );
+		$this->assertStringContains( 'notice-warning', get_echo( [ 'AMP_Options_Manager', 'insecure_connection_notice' ] ) );
+
+		$_SERVER['HTTPS'] = 'on';
+		$set_https_url    = static function ( $url ) {
+			return set_url_scheme( $url, 'https' );
+		};
+		add_filter( 'home_url', $set_https_url );
+		add_filter( 'site_url', $set_https_url );
+		set_current_screen( 'toplevel_page_' . AMP_Options_Manager::OPTION_NAME );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'insecure_connection_notice' ] ) );
+	}
+
+	/** @covers AMP_Options_Manager::reader_theme_fallback_notice() */
+	public function test_reader_theme_fallback_notice() {
+		$admin_user = self::factory()->user->create( [ 'role' => 'administrator' ] );
+		wp_set_current_user( $admin_user );
+		AMP_Options_Manager::update_options(
+			[
+				Option::THEME_SUPPORT => 'reader',
+				Option::READER_THEME  => 'foobar',
+			]
+		);
+
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'reader_theme_fallback_notice' ] ) );
+
+		set_current_screen( 'index' );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'reader_theme_fallback_notice' ] ) );
+
+		set_current_screen( 'themes' );
+		$this->assertStringContains( 'notice-warning', get_echo( [ 'AMP_Options_Manager', 'reader_theme_fallback_notice' ] ) );
+
+		set_current_screen( 'toplevel_page_' . AMP_Options_Manager::OPTION_NAME );
+		$this->assertStringContains( 'notice-warning', get_echo( [ 'AMP_Options_Manager', 'reader_theme_fallback_notice' ] ) );
+
+		AMP_Options_Manager::update_option( Option::READER_THEME, ReaderThemes::DEFAULT_READER_THEME );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'reader_theme_fallback_notice' ] ) );
+
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'foobar' );
+		wp_set_current_user( 0 );
+		$this->assertEmpty( get_echo( [ 'AMP_Options_Manager', 'reader_theme_fallback_notice' ] ) );
+	}
 }
diff --git a/tests/php/test-class-amp-reader-themes-rest-controller.php b/tests/php/test-class-amp-reader-themes-rest-controller.php
index 9ea5fbae6f1..8fcfc68a3e2 100644
--- a/tests/php/test-class-amp-reader-themes-rest-controller.php
+++ b/tests/php/test-class-amp-reader-themes-rest-controller.php
@@ -69,10 +69,13 @@ public function test_get_items() {
 			];
 		};
 
-		// Test that a theme with no screenshot_url is filtered out.
+		// Test that only the filtered and AMP Legacy themes are returned.
 		$this->controller = new AMP_Reader_Theme_REST_Controller( new ReaderThemes() );
 		add_filter( 'amp_reader_themes', $filter );
-		$this->assertEquals( 0, count( $this->controller->get_items( new WP_REST_Request( 'GET', 'amp/v1' ) )->data ) );
+
+		$data = $this->controller->get_items( new WP_REST_Request( 'GET', 'amp/v1' ) )->data;
+
+		$this->assertEquals( [ 'my-theme', 'legacy' ], wp_list_pluck( $data, 'slug' ) );
 		remove_filter( 'amp_reader_themes', $filter );
 	}
 }
diff --git a/tests/php/test-class-amp-template-customizer.php b/tests/php/test-class-amp-template-customizer.php
index 863af77b4ac..39b1a555d71 100644
--- a/tests/php/test-class-amp-template-customizer.php
+++ b/tests/php/test-class-amp-template-customizer.php
@@ -9,6 +9,7 @@
 use AmpProject\AmpWP\ReaderThemeLoader;
 use AmpProject\AmpWP\Services;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\AmpWP\Tests\Helpers\PrivateAccess;
 
 /**
@@ -19,10 +20,8 @@
 class Test_AMP_Template_Customizer extends WP_UnitTestCase {
 
 	use AssertContainsCompatibility;
-
 	use PrivateAccess;
-
-	private $original_theme_directories;
+	use LoadsCoreThemes;
 
 	public static function setUpBeforeClass() {
 		require_once ABSPATH . WPINC . '/class-wp-customize-manager.php';
@@ -32,18 +31,14 @@ public static function setUpBeforeClass() {
 	public function setUp() {
 		parent::setUp();
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	public function tearDown() {
 		parent::tearDown();
 		unset( $GLOBALS['wp_customize'], $GLOBALS['wp_scripts'], $GLOBALS['wp_styles'] );
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+
+		$this->restore_theme_directories();
 	}
 
 	/**
diff --git a/tests/php/test-includes-admin-functions.php b/tests/php/test-includes-admin-functions.php
index 65be158c369..288f8247d97 100644
--- a/tests/php/test-includes-admin-functions.php
+++ b/tests/php/test-includes-admin-functions.php
@@ -7,21 +7,29 @@
 
 use AmpProject\AmpWP\Admin\ReaderThemes;
 use AmpProject\AmpWP\Option;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 
 /**
  * Class Test_AMP_Admin_Includes_Functions
  */
 class Test_AMP_Admin_Includes_Functions extends WP_UnitTestCase {
 
+	use LoadsCoreThemes;
+
 	public function setUp() {
 		parent::setUp();
 		remove_all_actions( 'amp_init' );
 		remove_all_actions( 'admin_menu' );
 		remove_all_actions( 'customize_register' );
+
+		$this->register_core_themes();
 	}
 
 	public function tearDown() {
 		parent::tearDown();
+
+		$this->restore_theme_directories();
+
 		unset(
 			$GLOBALS['submenu'],
 			$GLOBALS['menu']
@@ -43,7 +51,7 @@ public function test_amp_init_customizer_legacy_reader() {
 	public function test_amp_init_customizer_modern_reader() {
 		switch_theme( 'twentytwenty' );
 		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
-		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentynineteen' );
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentyseventeen' );
 		$this->assertFalse( amp_is_legacy() );
 		amp_init_customizer();
 		$this->assertEquals( 500, has_action( 'customize_register', [ 'AMP_Template_Customizer', 'init' ] ) );
@@ -97,7 +105,7 @@ public function test_amp_add_customizer_link_reader_theme() {
 
 		wp_set_current_user( self::factory()->user->create( [ 'role' => 'administrator' ] ) );
 		AMP_Options_Manager::update_option( Option::THEME_SUPPORT, AMP_Theme_Support::READER_MODE_SLUG );
-		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentynineteen' );
+		AMP_Options_Manager::update_option( Option::READER_THEME, 'twentyseventeen' );
 		$this->assertFalse( amp_is_legacy() );
 		add_filter( 'amp_customizer_is_enabled', '__return_false' ); // This will be ignored.
 		amp_add_customizer_link();
diff --git a/tests/php/validation/test-class-amp-validated-url-post-type.php b/tests/php/validation/test-class-amp-validated-url-post-type.php
index 822ac646f79..95205be9a92 100644
--- a/tests/php/validation/test-class-amp-validated-url-post-type.php
+++ b/tests/php/validation/test-class-amp-validated-url-post-type.php
@@ -8,6 +8,7 @@
 use AmpProject\AmpWP\Option;
 use AmpProject\AmpWP\Tests\Helpers\AssertContainsCompatibility;
 use AmpProject\AmpWP\Tests\Helpers\HandleValidation;
+use AmpProject\AmpWP\Tests\Helpers\LoadsCoreThemes;
 use AmpProject\AmpWP\Tests\Helpers\PrivateAccess;
 
 // phpcs:disable WordPress.Variables.GlobalVariables.OverrideProhibited
@@ -22,18 +23,14 @@ class Test_AMP_Validated_URL_Post_Type extends WP_UnitTestCase {
 	use AssertContainsCompatibility;
 	use HandleValidation;
 	use PrivateAccess;
+	use LoadsCoreThemes;
 
 	const TESTED_CLASS = 'AMP_Validated_URL_Post_Type';
 
-	private $original_theme_directories;
-
 	public function setUp() {
 		parent::setUp();
 
-		global $wp_theme_directories;
-		$this->original_theme_directories = $wp_theme_directories;
-		register_theme_directory( ABSPATH . 'wp-content/themes' );
-		delete_site_transient( 'theme_roots' );
+		$this->register_core_themes();
 	}
 
 	public function tearDown() {
@@ -42,9 +39,7 @@ public function tearDown() {
 		global $current_screen;
 		$current_screen = null;
 
-		global $wp_theme_directories;
-		$wp_theme_directories = $this->original_theme_directories;
-		delete_site_transient( 'theme_roots' );
+		$this->restore_theme_directories();
 	}
 
 	/**