code: public function addCSS($css)
      {
          if (is_array($css)) {
              foreach ($css as $key => $value) {
                  if ($this->isLess($css)) {
                      $this->css[] = new FileAsset($css, array(LessphpFilter));
                  } elseif ($this->isScss($css)) {
                      $this->css[] = new FileAsset($css, array(ScssphpFilter));
                  } else {
                      $this->css[] = new FileAsset($css);
                  }
              }
              return $this;
          } else {
              throw new InternalException("CSS files to add is not an array");
          }
      }
docs: /*
  Public: Add css files
  
  $css - array of css files to load
  
  Returns css array