Hi Andreas Schramm,
Thank you for using Syncfusion Products.
We have checked the reported query regarding customization of TabControl and GroupBar.
Query #1: Customization of TabControl
You can customize the TabControl background color of header panel by changing the Background color property as shown in the following image.
Query #2: Customization of GroupBar
It can be done by changing the background of MainHost border . We are currently preparing the sample for the same. We would prepare and update you the complete details on 8th Oct 2019.
Regards,
Vijayalakshmi V.R.