Currently the embossing of UI widgets is hard-coded, often resulting in bad highlights/shadows in themes. It is particularly noticeable in dark themes.
Per @William Reynish (billrey)'s mockups in T38037, I think it's time we remove the embossing from UI widgets. This use to be an option in the theme preferences, but we are already overloaded with options.
I propose we remove embossing entirely, yielding a more modern, flat UI style that is more easily theme able and clean.
This is actually quite a subtle change in most places. For example, here is the default them with and without embossing:
The change is more noticeable on darker themes, such as Graph:
I am willing/able to undertake this myself. I've already been experimenting with the embossing code and it's not difficult. The main challenge will be cleaning up the various remnants of the now defunct emboss theme option.