Move styles submenu item into compat folder

lib/compat/wordpress-6.8/site-editor.php

@@ -122,3 +122,27 @@ function gutenberg_styles_wp_die_handler( $default_handler ) {
 	return $default_handler;
 }
 add_filter( 'wp_die_handler', 'gutenberg_styles_wp_die_handler' );
+
+/**
+ * Add a Styles submenu under the Appearance menu
+ * for Classic themes.
+ *
+ * @global array $submenu
+ */
+function gutenberg_add_styles_submenu_item() {
+	if ( ! wp_is_block_theme() ) {
+		global $submenu;
+
+		$styles_menu_item = array(
+			__( 'Design', 'gutenberg' ),
+			'edit_theme_options',
+			'site-editor.php',
+		);
+		// If $submenu exists, insert the Styles submenu item at position 2.
+		if ( $submenu && isset( $submenu['themes.php'] ) ) {
+			// This might not work as expected if the submenu has already been modified.
+			array_splice( $submenu['themes.php'], 1, 1, array( $styles_menu_item ) );
+		}
+	}
+}
+add_action( 'admin_init', 'gutenberg_add_styles_submenu_item' );