Hi Alyssa,
Yes, the problem here is that we have to have some programmatic logic in place to set the defaults for the FontFamily and FontSize, because the values we set can change based on a couple factors. Since we are setting it in code, it prevents the Style from being able to be applied.
What you could possibly do is make a class with an attached property that you set, and in the property changed handler for that, programmatically set the FontFamily and FontSize properties of the target object (the Ribbon). That would likely work around the problem, and that attached property could probably be set in your Style.