I’ve used dark theme on GitHub for several weeks, but than switched back to a light one. The reason for that is that I spend a lot of time writing and reading comments, and, in the dark theme, this is just hard on the eyes. I think part of the problem is
14 font size, which is just too small, but there might be some other typography tricks.
Note that I am a long-time user of dark themes in Discoures and Gmail, and they don’t suffer from this problem. Here are a couple of screenshots: