public function registerCoreCss($url = null, $media = 'screen')
    {
        if ($url === null) {
            $fileName = YII_DEBUG ? 'bootstrap.css' : 'bootstrap.min.css';
            $url = $this->getBootstrapUrl() . '/css/' . $fileName;
        }
        Yii::app()->getClientScript()->registerCssFile($url, $media);
    }