public function getDefaultTheme($target){
        $defaultTheme = null;
        if(!$this->isRegistered($target)){
            throw new \common_Exception('Target '.$target.' does not exist');
        } else {
            $array = $this->get($target);
            if(isset($array['default'])){
                $themeId = $array['default'];
                try{
                    $defaultTheme = $this->getTheme($target, $themeId);
                }catch(Exception $e){
                    //not found
                    $defaultTheme = null;
                }
            }
        }
        return $defaultTheme;
    }