private static function generateCSS( $force = false )
    {
        $cli = eZCLI::instance();
        $cli->output("\n Generating css files");

        foreach ( glob('extension/*/design/*/stylesheets/*.less') as $lessFilePath )
        {
            $lessFile = new KLessFile($lessFilePath);
            if ( $force )
            {
                $lessFile->compile();
            }
            else
            {
                $lessFile->compileIfNeeded();
            }
        }
    }
    /**
     * @param array     $lessFiles          less files to compile
     * @param string    $media
     * @param string    $cacheIdentifier    identifier used to prefix the cache entry for the packer
     */
    public static function generateCssTag( $lessFiles, $media, $cacheIdentifier = null )
    {
        if ( !is_array( $lessFiles ) )
        {
            $lessFiles = array( $lessFiles );
        }
        if ( is_null( $cacheIdentifier ) )
        {
            $lessFilePaths = array();
            foreach ( $lessFiles as $lessFile )
            {
                $kLessFile = new KLessFile( $lessFile );
                $lessFilePaths[] = $kLessFile->getCssFilePath();
            }
            $cacheIdentifier = md5(json_encode($lessFilePaths));
        }

        if ( empty($lessFiles) )
        {
            return;
        }

        $timestamp = 0;
        /** @var array(KLessFile) $kLessFiles */
        $kLessFiles = array();
        foreach ( $lessFiles as $lessFileName )
        {
            $kLessFile      = new KLessFile( $lessFileName );
            $timestamp      = max( $timestamp, filemtime($kLessFile->getLessFilePath()) );
            $kLessFiles[]   = $kLessFile;
        }

        if ( empty( $kLessFiles) )
        {
            return;
        }

        $packFilePath   = eZSys::cacheDirectory() . '/public/stylesheets/k' . $cacheIdentifier . '_' . $timestamp . '.css';
        $clusterFile    = eZClusterFileHandler::instance( $packFilePath );

        if ( !$clusterFile->exists() )
        {
            $css = '';
            foreach( $kLessFiles as $kLessFile )
            {
                $css .= $kLessFile->cssContent();
            }

            $clusterFile->storeContents( $css, 'klesspack', 'text/css' );
            KLessCache::cssPackCacheCleanUp( $cacheIdentifier );
        }

        $tag = '<link rel="stylesheet" type="text/css" href="/' . $packFilePath . '" ';
        if ( $media )
        {
            $tag .= 'media="'.$media.'" ';
        }
        $tag .= "/>\n";

        return $tag;
    }