Hi RiNDo
Thank you for contacting Syncfusion Support.
We have checked the reported case that theme for GroupBar header is not applied properly when the items populated using Prism. We have forwarded this to development team and they are currently validating it. We will check and update you the complete details by tomorrow [30th Sep, 2020]
Please let us know if you have any other concerns.
Regards,
Vijayalakshmi VR