We are using the theme per default. We did not configure that part. So probably this our problem. What is the resource to link to?
Here is what we have configure so far (this is really the basis):
<ribbon:Ribbon x:Name="ribbon" ApplicationButtonLabel="File"><!--The Project Menu Tab --><ribbon:Ribbon.ApplicationMenu><ribbon:Backstage Name="fltBackStage"><!--New Button--><ribbon:Button .../><ribbon:Button .../><ribbon:Separator /><ribbon:BackstageTab Header="Recent" KeyTipAccessText="R"><.../></ribbon:BackstageTab><ribbon:Button .../><ribbon:Button .../></ribbon:Backstage></ribbon:Ribbon.ApplicationMenu><!--Tabs--><ribbon:Ribbon.Tabs><.../></ribbon:Tab><ribbon:Ribbon.Tabs><.../></ribbon:Tab><ribbon:Ribbon.Tabs><.../></ribbon:Tab><ribbon:Ribbon.Tabs><.../></ribbon:Tab><ribbon:Ribbon.Tabs><.../></ribbon:Tab></ribbon:Ribbon.Tabs></ribbon:Ribbon>