public function add($file_path, $media = 'screen', $condition = null)
    {
        $_fp = $this->__template->findAsset($file_path);
        if ($_fp || \AssetsManager\Loader::isUrl($file_path)) {
            $this->registry->addEntry(array(
                'file'=>$_fp, 'media'=>$media, 'condition'=>$condition
            ), 'css_files');
        } else {
            throw new \InvalidArgumentException(
                sprintf('CSS file "%s" not found!', $file_path)
            );
        }
        return $this;
    }