Proxy\Plugin\ProxifyPlugin::css_url PHP Method

    private function css_url($matches)
    {
        $url = trim($matches[1]);
        if (stripos($url, 'data:') === 0) {
            return $matches[0];
        }
        return str_replace($matches[1], proxify_url($matches[1], $this->base_url), $matches[0]);
    }