private static function get_url_to_custom_style(&$stylesheet_urls)
 {
     $uploads = FrmStylesHelper::get_upload_base();
     $saved_css_path = '/formidable/css/formidablepro.css';
     if (is_readable($uploads['basedir'] . $saved_css_path)) {
         $url = $uploads['baseurl'] . $saved_css_path;
     } else {
         $url = admin_url('admin-ajax.php') . '?action=frmpro_css';
     }
     $stylesheet_urls['formidable'] = $url;
 }