(mvc) align theme selection a bit more

src/opnsense/mvc/app/controllers/OPNsense/Base/ControllerBase.php

@@ -213,7 +213,9 @@ class ControllerBase extends ControllerRoot
         $this->view->menuSystem = $menu->getItems("/ui".$this->router->getRewriteUri());
 
         // set theme in ui_theme template var, let template handle its defaults (if there is no theme).
-        if ($cnf->object()->theme != null && !empty($cnf->object()->theme)) {
+        if ($cnf->object()->theme != null && !empty($cnf->object()->theme) &&
+            is_dir('/usr/local/opnsense/www/themes/'.(string)$cnf->object()->theme)
+        ) {
             $this->view->ui_theme = $cnf->object()->theme;
         }