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