Index: wp-includes/class-wp-theme.php
===================================================================
--- wp-includes/class-wp-theme.php	(revision 21157)
+++ wp-includes/class-wp-theme.php	(working copy)
@@ -1071,7 +1071,7 @@
 	 * @return array Array of stylesheet names.
 	 */
 	public static function get_allowed( $blog_id = null ) {
-		return self::get_allowed_on_network() + self::get_allowed_on_site( $blog_id );
+		return apply_filters( 'allowed_themes', self::get_allowed_on_network() + self::get_allowed_on_site( $blog_id ) );
 	}
 
 	/**