Hi Joris,
You can use something like that inside your template :
$remove_client_css   =   $this->getStyleParam( 'remove_client_css', 1 );
     $remove_cck_css     =   $this->getStyleParam( 'remove_cck_css', 1 );
     $add_custum_css     =   $this->getStyleParam( 'add_custum_css', 0 );         
     $doc         =   JFactory::getDocument();
     $headData       =   $doc->getHeadData();
     
     if ( $remove_client_css ) {
       unset($headData['styleSheets'][JURI::root( true ).'/media/cck/css/cck.'.$this->client.'.css']);
     }
     if ( $remove_cck_css ) {
       unset($headData['styleSheets'][JURI::root( true ).'/media/cck/css/cck.css']);
     }
     if(!count($headData['styleSheets']))
       $headData['styleSheets'] = array('data:text/css,'=>array('mime'=>'text/css'));
     $doc->setHeadData($headData); 
     
     if ( $add_custum_css ) {
       $css_override   =   $this->path.'/css/custum.css';
       if ( $this->isFile( $css_override ) ) {
         $doc->addStyleSheet( JURI::root( true ).'/templates/'.$this->name. '/css/custum.css' );
       }
     }
Best regards.