Neos\Neos\Utility\BackendAssetsUtility::getCssBuiltVersion PHP Method

    public function getCssBuiltVersion()
    {
        return substr(md5_file('resource://Neos.Neos/Public/Styles/Includes-built.css'), 0, 12);
    }

 /**
  * @return string
  */
 public function render()
 {
     return $this->backendAssetsUtility->getCssBuiltVersion();
 }