There are some limitations of what we can do with FLTK schemes.
[ . . . ]
I've been working on this and my basic API design is a base class
Fl_Scheme with derived classes
[ . . . ]
I got the basics working but there's still much to do before I can
provide a fully working prototype.
Just an observation. I have the feeling this isn't trivial :-(
I've been obliged to look into switching from default black text on a
white background this week, and found that it's not simple even on
systems that offer accessibility settings.
On Windows10 I've set the desktop as black, and have gone for the
High Contrast Dark# scheme, which gives green text on black, but
so many applications don't honour the change or have colours that
don't work. One app switched to yellow text on a white background.
On the Mac, famed for its usability, the high-level reverse-video just
reverses all colours, including in images. Mostly usable, but maybe
I missed something.
As for my Linux box, there are options for Xfce borders, etc, but any
older X11 applications are on a case by case, option by option basis.
xterm -rv gives white text on black, but once anything adds syntax
directed colours, you end up with dark blue or red on black...