Hi Christophe,
Thank you for contacting Syncfusion support.
We have prepared a sample based on your requirement. In this sample, we have customized the Office2010Silver theme as RibbonBar style, where we set the Background color as Red for the RibbonBar's Header TextBlock. You can download the sample from the below link.
Sample:
RibbonBarHeaderColorSampleRegards,
Venkateshwaran V.R.