Hi Falk,
I apologize for the inconvenience caused.
I have opened a feature request (feature# 540)regarding this .We will try our best to implement this at the earliest.
You can track the status of feature here.
Feature Request #540 - Allow Checkboxes in TreeNodeAdv to have a Flat Style. .
We will inform you as soon as this feature request is addressed.
Thank you for your patience.
Best Regards,
Deepa.T.S