diff --git a/plugin/inc/functions.php b/plugin/inc/functions.php
index 75daf434..3f710115 100644
--- a/plugin/inc/functions.php
+++ b/plugin/inc/functions.php
@@ -17,6 +17,15 @@
 function plugin() {
 	static $plugin = null;
 
+	/**
+	 * Check if the requirements for the current plugin are met.
+	 * If the requirements are not met, we might get severe errors. Therefore, we
+	 * return null and do not initialize the plugin.
+	 */
+	if ( ! plugin_requirements_are_met() ) {
+		return null;
+	}
+
 	if ( null === $plugin ) {
 		/**
 		 * The main plugin component.
@@ -45,3 +54,43 @@ function plugin_container() {
 
 	return $container;
 }
+
+/**
+ * Check if the requirements for the current plugin are met.
+ *
+ * @return bool True if requirements are met, false otherwise.
+ */
+function plugin_requirements_are_met() {
+	/**
+	 * Advanced Custom Fields Pro is required.
+	 */
+	if ( ! function_exists( '\acf_add_options_page' ) ) {
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * Display an admin notice if the requirements are not met.
+ */
+function plugin_requirements_notice__error() {
+	if ( plugin_requirements_are_met() ) {
+		return;
+	}
+
+	?>
+	<div class="notice notice-error">
+		<p>
+			<?php
+			printf(
+				/* translators: %s: Plugin name. */
+				esc_html__( 'The requirements for the %s plugin are not met. Please install all requirements.', 'lhpbp' ),
+				'<strong>' . esc_html__( 'LHPBP Plugin', 'lhpbp' ) . '</strong>'
+			);
+			?>
+		</p>
+	</div>
+	<?php
+}
+add_action( 'admin_notices', '\WpMunich\lhpbp\plugin\plugin_requirements_notice__error' );
diff --git a/plugin/lhpbpp.php b/plugin/lhpbpp.php
index a6da0f36..8a26c9b0 100644
--- a/plugin/lhpbpp.php
+++ b/plugin/lhpbpp.php
@@ -15,6 +15,7 @@
  */
 
 use function WpMunich\lhpbp\plugin\plugin;
+use function WpMunich\lhpbp\plugin\plugin_requirements_are_met;
 
 // If this file is called directly, abort.
 if ( ! defined( 'WPINC' ) ) {
@@ -41,7 +42,7 @@
 call_user_func( 'WpMunich\lhpbp\plugin\plugin' );
 
 // Initialize the plugin update checker.
-if ( class_exists( 'Puc_v4_Factory' ) ) {
+if ( class_exists( 'Puc_v4_Factory' ) && plugin_requirements_are_met() ) {
 	Puc_v4_Factory::buildUpdateChecker(
 		'https://www.luehrsen-heinrich.de/updates/?action=get_metadata&slug=' . plugin()->get_plugin_slug(),
 		__FILE__, // Full path to the main plugin file or functions.php.
diff --git a/schemas/theme.json b/schemas/theme.json
index 44572bf8..6bc7f266 100644
--- a/schemas/theme.json
+++ b/schemas/theme.json
@@ -463,9 +463,31 @@
                     "px",
                     "em",
                     "rem",
-                    "vh",
+                    "%",
                     "vw",
-                    "%"
+                    "svw",
+                    "lvw",
+                    "dvw",
+                    "vh",
+                    "svh",
+                    "lvh",
+                    "dvh",
+                    "vi",
+                    "svi",
+                    "lvi",
+                    "dvi",
+                    "vb",
+                    "svb",
+                    "lvb",
+                    "dvb",
+                    "vmin",
+                    "svmin",
+                    "lvmin",
+                    "dvmin",
+                    "vmax",
+                    "svmax",
+                    "lvmax",
+                    "dvmax"
                   ],
                   "default": "rem"
                 }
diff --git a/theme/inc/functions.php b/theme/inc/functions.php
index 9915eb26..ba993137 100644
--- a/theme/inc/functions.php
+++ b/theme/inc/functions.php
@@ -37,6 +37,7 @@ function theme() {
 
 /**
  * Check if the requirements for the current theme are met.
+ * The only requirement for the theme is the accompanying plugin.
  *
  * @return bool True if requirements are met, false otherwise.
  */
@@ -48,13 +49,6 @@ function theme_requirements_are_met() {
 		return false;
 	}
 
-	/**
-	 * The Advanced Custom Fields plugin is required.
-	 */
-	if ( ! function_exists( 'get_field' ) ) {
-		return false;
-	}
-
 	return true;
 }
 
@@ -63,7 +57,7 @@ function theme_requirements_are_met() {
  */
 function requirements_template() {
 	if ( ! theme_requirements_are_met() ) {
-		wp_die( 'The requirements for this theme are not met.' );
+		wp_die( 'The requirements for this theme are not met. Please install and activate the accompanying plugin.' );
 	}
 }
 add_action( 'template_redirect', '\WpMunich\lhpbp\theme\requirements_template' );
@@ -77,7 +71,7 @@ function theme_requirements_notice__error() {
 	}
 
 	$class   = 'notice notice-error';
-	$message = 'The requirements for this theme are not met.';
+	$message = 'The requirements for this theme are not met. Please install and activate the accompanying plugin.';
 
 	printf( '<div class="%1$s"><p>%2$s</p></div>', esc_attr( $class ), esc_html( $message ) );
 }