When an ApplicationMenu2010Item is selected it shows a horizontal triangle on its right edge. By default its a white triangle which blends into the white background used by its Content. We are not using a white background for our content but the triangle on the ApplicationMenu2010Item is still white. So we see the selected ApplicationMenu2010Item shown with its ApplicatoinAccentColor and a white triangle on its right edge next to its dark gray content background. I have attached an image that I added a red square around the white triangle on the ApplicationMenu2010Item to help explain what I am talking about. In this image the white triangle blends perfectly into the white background of its content.
Can I style the color of the white triangle for the selected ApplicationMenu2010Item so it blends into the background of our content?
Ok I can see the ControlTemplate for igRibbon:ApplicationMenu2010Item in ../WPF/DefaultStyles/Ribbon/RibbonGeneric.xaml and it has a <Path> for the triangle and its Fill is set to White. I do not see a way to override this without recreating the whole ControlTemplate.
There is no property that determines just that color as you noticed. Rather than hard coding the color to White in the template we should probably bind that to the Background of the ancestor ApplicationMenu2010 so if the Background of the backstage is changed it will at least be in sync with that. We can make that change although I'm not sure that helps with what you are looking for. At least at that point you would be able to bind the Background of the ApplicationMenu2010 to a property (perhaps you're own attached property) of the SelectedTab. I don't think we would add a brush or property for this as this triangle is specific to Office 2010 - Office 2013 doesn't display that wedge.