The bad news: Dark Mode will be Stack Overflow only for at least a while:
At this point, the focus of Dark Mode is on Stack Overflow and we’ll eventually bring it to MSO. While the retheming we did across the network two years ago makes updating our LESS easier, creating dark versions of all of our sites, particularly the custom-designed ones, is going to be a huge challenge we’re not able to contemplate at this time. The artwork on some of those sites simply can’t be made dark because we don’t have access to the original art files and, to be honest, some of the themes will always be better as-is.
The good news: when it will be available, sites without a design will probably be first:
From talking to Aaron... it's kinda complicated. We can't just turn a lot of sites dark because their artwork was designed with light mode in mind (excepting for Science Fiction & Fantasy, for the most part). So plopping dark mode on, say... Worldbuilding or English Language Learners or Seasoned Advice... would be pretty impossible without redoing their artwork... again. This is one of those... rare cases where beta sites have a leg up... :)
That is of course assuming that Drones.SE doesn't grow fast enough to leave beta and get a custom design (there's quite a long waiting list for that) before Dark Mode is rolled out to other sites.