Try: Add buttons styles.

assets/css/button-outline.css

@@ -0,0 +1,6 @@
+.wp-block-button.is-style-outline
+	> .wp-block-button__link:not(.has-text-color, .has-background):hover {
+	background-color: var(--wp--preset--color--contrast, transparent);
+	color: var(--wp--preset--color--base);
+	border-color: var(--wp--preset--color--contrast, currentColor);
+}

functions.php

@@ -35,3 +35,28 @@ function twentytwentyfive_enqueue_styles() {
 	);
 }
 add_action( 'wp_enqueue_scripts', 'twentytwentyfive_enqueue_styles' );
+
+/**
+ * Enqueue custom block stylesheets
+ *
+ * @since Twenty Twenty-Five 1.0
+ * @return void
+ */
+function twentytwentyfive_block_stylesheets() {
+	/**
+	 * The wp_enqueue_block_style() function allows us to enqueue a stylesheet
+	 * for a specific block. These will only get loaded when the block is rendered
+	 * (both in the editor and on the front end), improving performance
+	 * and reducing the amount of data requested by visitors.
+	 */
+	wp_enqueue_block_style(
+		'core/button',
+		array(
+			'handle' => 'twentytwentyfive-button-style-outline',
+			'src'    => get_parent_theme_file_uri( 'assets/css/button-outline.css' ),
+			'ver'    => wp_get_theme( get_template() )->get( 'Version' ),
+			'path'   => get_parent_theme_file_path( 'assets/css/button-outline.css' ),
+		)
+	);
+}
+add_action( 'init', 'twentytwentyfive_block_stylesheets' );

theme.json

@@ -931,6 +931,35 @@
 			"lineHeight": "1.4"
 		},
 		"blocks": {
+			"core/button": {
+				"typography": {
+					"fontSize": "var:preset|font-size|small"
+				},
+				"spacing": {
+					"padding": {
+						"bottom": "1rem",
+						"left": "2.25rem",
+						"right": "2.25rem",
+						"top": "1rem"
+					}
+				},
+				"variations": {
+					"outline": {
+						"border": {
+							"color": "var:preset|color|contrast",
+							"width": "1px"
+						},
+						"spacing": {
+							"padding": {
+								"bottom": "1rem",
+								"left": "2.25rem",
+								"right": "2.25rem",
+								"top": "1rem"
+							}
+						}
+					}
+				}
+			},
 			"core/code": {
 				"typography": {
 					"fontFamily": "var:preset|font-family|fira-code",
@@ -1111,20 +1140,42 @@
 		},
 		"elements": {
 			"button": {
-				"typography": {
-					"fontSize": "var:preset|font-size|small"
-				},
 				"color": {
 					"background": "var:preset|color|contrast",
 					"text": "var:preset|color|base"
 				},
-				"spacing": {
-					"padding": {
-						"bottom": "1rem",
-						"left": "2.25rem",
-						"right": "2.25rem",
-						"top": "1rem"
+				":active": {
+					"color": {
+						"background": "var:preset|color|accent-1",
+						"text": "var:preset|color|contrast"
+					}
+				},
+				":focus": {
+					"color": {
+						"background": "var:preset|color|accent-1",
+						"text": "var:preset|color|contrast"
+					},
+					"outline": {
+						"color": "var:preset|color|primary",
+						"offset": "2px",
+						"style": "dotted",
+						"width": "1px"
 					}
+				},
+				":hover": {
+					"color": {
+						"background": "var:preset|color|accent-1",
+						"text": "var:preset|color|contrast"
+					},
+					"border": {
+						"color": "var:preset|color|accent-1"
+					}
+				},
+				"border": {
+					"color": "var:preset|color|contrast",
+					"radius": "100px",
+					"width": "1px",
+					"style": "solid"
 				}
 			},
 			"caption": {