code: public function updateStyleSheet()
  	{
  		/** @var Symfony\Component\HttpFoundation\Session\SessionInterface $objSession */
  		$objSession = Contao\System::getContainer()->get('session');
  
  		if ($objSession->get('style_sheet_update_all'))
  		{
  			$this->import('Contao\StyleSheets', 'StyleSheets');
  			$this->StyleSheets->updateStyleSheets();
  		}
  
  		$objSession->set('style_sheet_update_all', null);
  	}
docs: Check for modified style sheets and update them if necessary