diff --git a/src/wp-includes/default-filters.php b/src/wp-includes/default-filters.php
index 126d276417..725f5ef3cb 100644
--- a/src/wp-includes/default-filters.php
+++ b/src/wp-includes/default-filters.php
@@ -557,8 +557,10 @@ add_action( 'wp_default_scripts', 'wp_default_packages' );
 
 add_action( 'wp_enqueue_scripts', 'wp_localize_jquery_ui_datepicker', 1000 );
 add_action( 'wp_enqueue_scripts', 'wp_common_block_scripts_and_styles' );
+add_action( 'wp_enqueue_scripts', 'wp_enqueue_classic_theme_styles' );
 add_action( 'admin_enqueue_scripts', 'wp_localize_jquery_ui_datepicker', 1000 );
 add_action( 'admin_enqueue_scripts', 'wp_common_block_scripts_and_styles' );
+add_action( 'admin_enqueue_scripts', 'wp_enqueue_classic_theme_styles' );
 add_action( 'enqueue_block_assets', 'wp_enqueue_registered_block_scripts_and_styles' );
 add_action( 'enqueue_block_assets', 'enqueue_block_styles_assets', 30 );
 add_action( 'enqueue_block_editor_assets', 'wp_enqueue_registered_block_scripts_and_styles' );
diff --git a/src/wp-includes/script-loader.php b/src/wp-includes/script-loader.php
index 3bd0bbcaf2..0c64281843 100644
--- a/src/wp-includes/script-loader.php
+++ b/src/wp-includes/script-loader.php
@@ -3640,3 +3640,16 @@ function _wp_theme_json_webfonts_handler() {
 	add_action( 'wp_enqueue_scripts', $fn_generate_and_enqueue_styles );
 	add_action( 'admin_init', $fn_generate_and_enqueue_editor_styles );
 }
+
+/**
+ * Loads classic theme styles on classic themes.
+ *
+ * This is needed for backwards compatibility for button blocks specifically.
+ */
+function wp_enqueue_classic_theme_styles() {
+	if ( ! wp_is_block_theme() ) {
+		$suffix = wp_scripts_get_suffix();
+		wp_register_style( 'classic-theme-styles', "/wp-includes/css/dist/block-library/classic$suffix.css", array(), true );
+		wp_enqueue_style( 'classic-theme-styles' );
+	}
+}