get_theme_path: fallback to css/default.css if default theme is selected

include/functions.php

@@ -2425,6 +2425,9 @@
 	}
 
 	function get_theme_path($theme) {
+		if ($theme == "default.php")
+			return "css/default.css";
+
 		$check = "themes/$theme";
 		if (file_exists($check)) return $check;