core-bundle/src/Resources/contao/dca/tl_theme.php
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);
	}
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);
	}
Check for modified style sheets and update them if necessary