code: public function scheduleUpdate()
  	{
  		// Return if there is no ID
  		if (!CURRENT_ID || Contao\Input::get('act') == 'copy')
  		{
  			return;
  		}
  
  		/** @var Symfony\Component\HttpFoundation\Session\SessionInterface $objSession */
  		$objSession = Contao\System::getContainer()->get('session');
  
  		// Store the ID in the session
  		$session = $objSession->get('style_sheet_updater');
  		$session[] = CURRENT_ID;
  		$objSession->set('style_sheet_updater', array_unique($session));
  	}
docs: Schedule a style sheet update
  
  This method is triggered when a single style or multiple styles are
  modified (edit/editAll), duplicated (copy/copyAll), moved (cut/cutAll)
  or deleted (delete/deleteAll).