You can do something like this:
var theme = get_theme()
var newfont = load('assets/fonts/different-font.fnt')
I haven't confirmed whether this works on controls that just inherit a theme; if it doesn't, just call
get_theme() on the themed parent control instead.
(Also, if all you're changing is the size, you can probably refine this to use DynamicFont and DynamicFontData.)