Recently, dark mode has been applied on https://stackoverflow.com.
The result is fantastic to me. Is there any plan to bring it to MSE?
It would appear that it is not: Introducing Dark Mode (beta) for Stack Overflow.
There are a lot of challenges to implementing dark mode across a platform like ours. We are among the 40 largest websites in the world in terms of monthly unique visitors. We also have hundreds of communities with different design aesthetics. On top of our public Q&A platform, we also have our developer profiles, job listings, product pages, and many other surfaces.
Despite the challenges, this opportunity was too big to be ignored. Today [30 March 2020], we are proud to announce the release of Dark Mode in beta for Stack Overflow. We want to share it with our community, especially our power users, and gather feedback so we can improve, iterate, and expand Dark Mode in the future. You can opt-in Dark Mode through your user preferences. You must be logged into your Stack Overflow account to get this option.
For now, we have no plans to bring dark mode to the many sites across the Stack Exchange network. Many of the designs on our sites have been around long enough that converting them to dark mode would require redoing the artwork completely. We would prefer to avoid giving anyone across our network a substandard experience and we don’t want to change those elements without the input of these communities.
My reading of the blog post in general suggests that they did it because many programmers have Stack Overflow open literally pretty much all day. Programmers often have dark modes for their IDE (integrated development environment) of choice.
This is not the case for other SE sites. Some people may use SE Maths a lot, but it's not open 8hrs per day (except, perhaps, for a small minority). Even professional mathematicians who would use Maths Stack Exchange a lot (who may well use Math Overflow quite a lot too), they spend a lot of time writing on paper (or similar).
It seems to be that the argument is that the need is much lower for other sites, and they say that there would be significant work in changing them -- cue excuse about "not wanting to give anyone... a substandard experience" to get out of doing it!