Anyway, returning to the original question, a flat style is actually pretty easy to make in fltk since the stock widgets all support a FL_FLAT_BOX style.

Yes, and I found in fluid you can set a button's Box: to Boxes -> RFLAT_BOX to get a round-edged button that, when colored blue, can look like this in an excerpt of an application screenshot:

And in the above, the "rounded frame" is just an Fl_Box with the Box: set to Frames -> ROUNDED_FRAME in fluid,
and the "Upgrade Firmware" that interrupts the frame line is just an Fl_Box over it that has the Box: set to boxes -> FLAT_BOX
with a color matching the window's background color (which has been made an almost-white).

That's all pretty trivial styling just with stock widgets. I didn't change the Fl::scheme() at all. For flat widgets, the default
FLTK scheme is best, since it doesn't introduce any gradients and such that would affect the flat look.