diff --git a/calendar/inc/class.calendar_ui.inc.php b/calendar/inc/class.calendar_ui.inc.php
index 2e261d8c7f..ce77c5c5d4 100644
--- a/calendar/inc/class.calendar_ui.inc.php
+++ b/calendar/inc/class.calendar_ui.inc.php
@@ -238,8 +238,9 @@ class calendar_ui
 		// load etemplate2
 		egw_framework::validate_file('/etemplate/js/etemplate2.js');
 
-		// load our app.js file
-		egw_framework::validate_file('/calendar/js/app.js');
+		// search for app specific css file, so it can customize the theme
+		egw_framework::includeCSS('calendar', 'app-'.$GLOBALS['egw_info']['user']['preferences']['common']['theme']) ||
+			egw_framework::includeCSS('calendar', 'app');
 
 		common::egw_header();