getMergedCssUrl

     public function getMergedCssUrl($files) {

        $tstamp = $this->getYoungestFile($files);

        // secure or unsecure
        $isSecure = Mage::app()->getRequest()->isSecure();
        $mergerDir = $isSecure ? 'css_secure' : 'css';
        $targetDir = $this->_initMergerDir($mergerDir);
        if (!$targetDir) {
            return '';
        }

        // base hostname & port
        $baseMediaUrl = Mage::getBaseUrl('media', $isSecure);
        $hostname = parse_url($baseMediaUrl, PHP_URL_HOST);
        $port = parse_url($baseMediaUrl, PHP_URL_PORT);
        if (false === $port) {
            $port = $isSecure ? 443 : 80;
        }

        // merge into target file
        $targetFilename = md5(implode(',', $files) . "|{$hostname}|{$port}") . '.' . $tstamp . '.css';
        if ($this->_mergeFiles($files, $targetDir . DS . $targetFilename, false, array($this, 'beforeMergeCss'), 'css')) {
            return $baseMediaUrl . $mergerDir . '/' . $targetFilename;
        }
        return '';
     }