Close icon next to the Help button

Hello,

I am trying to mimic the behavior seen in MS Excel by adding a close [x] button next to the help button in the ribbon. Is there native support for this behavior? Currently I have added a button to the TabItemToolbar to mimic this behavior, however the button looks pretty bad, since I cannot find any buttons that play together with all of the themes. Do you guys have a solution to this? I noticed some of the other vendors support this natively. Or at least the a set of buttons I can use that will work with all of the themese?

Thanks,
Brette