Thanks for your patience.
There are several changes that you need to do in order to make the required look:
1. you'll need implicit style for the GroupChrome where you set Background property:
2. Then you'll need to set Background to the RibbonView control.
3. Finally you'll need implicit style for the RadTabControl where you either set the OuterBorder element to be templatebound to the RibbonView Background or just hardcode the Background by yourself. Let me know if you need help with any of the steps suggested.
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