Use always white as the background color in HelpViewer

Our css styling assumes white as the background color.

Change-Id: Ieeddf1eba5bb29cadf0f4da7c8ee6b14c5d1387e
Task-number: QTCREATORBUG-13300
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>
QmlDesigner: Crashfix

we have to check if there is a valid kit. Otherwise we use the
fallback puppet.

Change-Id: Id9682542d2961a616029fad57c2dc57b827c7605
Task-number: QTCREATORBUG-13350
Reviewed-by: Alessandro Portale <alessandro.portale@digia.com>