Fl::background(r,g,b) changes the general "gray" color and the colorramp used for all the button edges
Fl::foreground(r,g,b) changes the color all the text and labels are drawn
Fl::background2(r,g,b) changes the "white" in the input fields
Fl::set_color(x,r,g,b) sets some other colors:
x = FL_INACTIVE_COLOR text color for inactive widgets
x = FL_SELECTION_COLOR background for text selection (text color is computed to contrast with it)
All of this was designed for mid-1990's style user preferences, but it still could be useful.