I came here to report this as a bug as well. This is clearly very inconsistent as already pointed out by several posters before me:
* Dark titlebar menus are while
* Desktop menus are white, even if the background is dark
* Custom themes are obviously not supported in third-party software, breaking the design decision early on
* Dark toolbar menus in nautilus are white
I'd be very grateful if someone can point me to the commit(s) where this was introduced. I am more than happy to revert locally and compile a more 'pleasing' and consistent theme for myself. Any tips how this 'surprisingly easy' task can be accomplished would be very welcomed.
Having a mixture of white/dark menus which try to follow some well-defined rules, but fail at every most case is not what I want for my everyday OS.