Reload CSS after login because the user may have a different theme (WIP)

This is just a quick fix.

refs #10957

library/Icinga/Authentication/Auth.php

@@ -135,6 +135,8 @@ class Auth
         } else {
             $preferences = new Preferences();
         }
+        // TODO(el): Quick-fix for #10957. Only reload CSS if the theme changed.
+        $this->getResponse()->setReloadCss(true);
         $user->setPreferences($preferences);
         $groups = $user->getGroups();
         foreach (Config::app('groups') as $name => $config) {