I just had a quick look at
http://www.fltk.org and see that one of those (^) is OS X (and thus, all three BMX platforms). There's also this, which sounds pretty good:
FLTK 1.1.x offers two appearance schemes: "standard" and "plastic". FLTK 2.0 will extend this to include platform schemes as well as user-defined schemes. Colors and fonts automatically adjust to the current user preferences.
I don't know which version the BlitzMax GUI will be using, but I like the sound of 2.0's user-defined schemes....