diff --git a/wp-includes/class-wp-theme.php b/wp-includes/class-wp-theme.php
index e099f39ba3..8be643327b 100644
--- a/wp-includes/class-wp-theme.php
+++ b/wp-includes/class-wp-theme.php
@@ -1071,7 +1071,8 @@ final class WP_Theme implements ArrayAccess {
 	 * @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 );
+		$network = (array) apply_filters( 'allowed_themes', self::get_allowed_on_network() );
+		return $network + self::get_allowed_on_site( $blog_id );
 	}
 
 	/**