Interesting thread - I've just stumbled onto it.
If there is interest, I've edited the base FLWM code to support using/reading a conf file (~/.flwm.conf). I use it for modifying key bindings and desktop names without rebuilding. Adding support for "themes" - frame and button color/width/size/style, etc., shouldn't be too hard. Main problem, of course, is finding time to add that code. It's a reasonably small change, too. I think 20k. Not 100k.
I'd be interested to see this thread fix/describe the palette (more specific than just stating blue/gray): RGBA or CMYK values for the "stock" set of gray/orange/black/white/blue. Then themers and icon designers can depart from that set. Stating the stock sizes may help. I'm not even sure what wbar wants.
I, for one, like the idea that FLWM is the base WM. I also like the lack of boot splash, but I do use quiet in grub.