Well I have tried changing that one in the past and it worked for the most part but for some reason it made the text on some buttons very small and unusable.

That location was: Buttons, Boxes & Controls > Buttons > Button > PushButton > Font:Font

The one I just tried that also seems to work is: Styles > ButtonStyle > PushButton > Font:Font

I am not sure if the same problems will occur using that location though as I do not remember exactly what buttons were effected last time.