code: public static function generateStyleTag($href, $media=null, $mtime=false)
  	{
  		// Add the filemtime if not given and not an external file
  		if ($mtime === null && !preg_match('@^https?://@', $href))
  		{
  			$container = System::getContainer();
  			$rootDir = $container->getParameter('kernel.project_dir');
  
  			if (file_exists($rootDir . '/' . $href))
  			{
  				$mtime = filemtime($rootDir . '/' . $href);
  			}
  			else
  			{
  				$webDir = StringUtil::stripRootDir($container->getParameter('contao.web_dir'));
  
  				// Handle public bundle resources in web/
  				if (file_exists($rootDir . '/' . $webDir . '/' . $href))
  				{
  					$mtime = filemtime($rootDir . '/' . $webDir . '/' . $href);
  				}
  			}
  		}
  
  		if ($mtime)
  		{
  			$href .= '?v=' . substr(md5($mtime), 0, 8);
  		}
  
  		return '<link rel="stylesheet" href="' . $href . '"' . (($media && $media != 'all') ? ' media="' . $media . '"' : '') . '>';
  	}
docs: Generate the markup for a style sheet tag
  
  @param string $href  The script path
  @param string $media The media type string
  @param mixed  $mtime The file mtime
  
  @return string The markup string