In our RibbonWindow we don’t have direct property to change the background but we can change by using the RibbonWindow template.
We have prepared a sample for the same in which we have changed the RibbonWindow Background using the RibbonWindow Template in this sample we have used two ColorPickerPalette to change the BackStage color and RibbonWindow Background. Please download the sample from the following location.
Sample link: RibbonSample.zip
Please let us know if you need further assistance.