code: public function checkCategory($varValue)
  	{
  		// Do not change the value if it has been set already
  		if (\strlen($varValue) || Contao\Input::post('FORM_SUBMIT') == 'tl_style')
  		{
  			return $varValue;
  		}
  
  		/** @var Symfony\Component\HttpFoundation\Session\Attribute\AttributeBagInterface $objSessionBag */
  		$objSessionBag = Contao\System::getContainer()->get('session')->getBag('contao_backend');
  
  		$key = 'tl_style_' . CURRENT_ID;
  		$filter = $objSessionBag->get('filter');
  
  		// Return the current category
  		if (\strlen($filter[$key]['category']))
  		{
  			return $filter[$key]['category'];
  		}
  
  		return '';
  	}
docs: Automatically set the category if not set
  
  @param mixed $varValue
  
  @return string