(I'll keep this post updated with stuff as I find it)
Some other posts have touched on this:
https://connect.mozilla.org/t5/ideas/transparent-context-menus/idi-p/8002
https://connect.mozilla.org/t5/ideas/give-to-context-menu-same-design-as-menus/idi-p/7318
Having a look at Firefox Color (create a custom theme) which looks great! However it would be even better if we could:
I think themes are underrated on FIrefox, they're already more flexible and customisable than Chrome/Edge. Keep up the good work!