public static function outputStyles($output = '')
    {
        self::lookIfProcessFiles('style', 'header');

        $template = self::$templates['style'];
        $styles = self::$data['style']['header'];

        foreach ($styles as $value) {
            $output .= sprintf(
                $template,
                $value['url']
            );
        }

        self::$data['style']['header'] = [];

        return ! empty($output) ? $output : false;
    }