If you use Stylish (and if not, why aren’t you? Get it for Firefox and Chrome) and like dark global themes, you might want to check out my global dark web theme. It’s a fork (meaning I’m starting with someone else’s template), but I’m slowly making it my own and keeping it updated. And this is also the style I use on a regular basis, so things that are irritating are gonna get fixed.
You can also exclude sites; I’ve excluded several by default (mainly Google and RTM) because their UI changes too frequently for me to keep them in track as well. If you’re using the new Remember the Milk UI and want a dark theme for that, I’ve got you covered there as well with Darker Remember The Milk. I also use Remember The Milk for my chores and stuff on a daily basis, so this gets updated when things break.