I'm wondering if the dark mode will be coming to other Stack Exchange sites, including all meta sites.
Sadly, the answer is either "No" or "Maybe":
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.
To me it looks pretty unlikely because every graduated site has its own design elements/stylesheets that may not go too well with one DM for all the sites. As for the Metas, it does say they'll eventually bring it to Meta Stack Overflow (I asked when), but not MSE.
I do agree with Catija - Seasoned Advice might not look too good with a universal Dark Mode, but even so, I'd rather see light on dark any day. I guess we'll have to wait and see...