Hi Bobby,
Thanks for letting us know about this. We've fixed it for the next version. In the meantime, if you redefine these brush resources in your app's Resources (per the techniques described in the Themes documentation and samples) as Transparent, you can get it looking how you want.
<SolidColorBrush x:Key="{x:Static themes:AssetResourceKeys.RibbonTabItemBackgroundHoverBrushKey}" themes:TintGroup.Name="RibbonTabItem" Color="#00FFFFFF" /><SolidColorBrush x:Key="{x:Static themes:AssetResourceKeys.RibbonTabItemBorderHoverBrushKey}" themes:TintGroup.Name="RibbonTabItem" Color="#00FFFFFF" />