draw_box(boxtype[0], xx[0], y(), ww[0], h(), FL_BACKGROUND2_COLOR);
So... well, you could try changing FL_BACKGROUND2_COLOR but that might have "unhelpful" side effects in other widgets.
Or copy/paste the Fl_Counter code and modify the widget to take a user selectable background colour in place of the hard-coded FL_BACKGROUND2_COLOR.