(mvc) fix handling of empty theme tag, issue https://github.com/opnsense/core/issues/342

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

@@ -212,8 +212,8 @@ class ControllerBase extends ControllerRoot
 
         $this->view->menuSystem = $menu->getItems("/ui".$this->router->getRewriteUri());
 
-        // set theme in ui_theme template var
-        if ($cnf->object()->theme != null) {
+        // set theme in ui_theme template var, let template handle its defaults (if there is no theme).
+        if ($cnf->object()->theme != null && !empty($cnf->object()->theme)) {
             $this->view->ui_theme = $cnf->object()->theme;
         }