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