I couldn't find whether this question has been already asked and whether this is really necessary.
Do we have an option to use dark theme in MSE? I am not aware whether MSE offers dark mode. It will be nice if dark mode is available.
Dark mode is only available for Stack Overflow.
There are pending feature requests and discussions on the main Meta see e.g. Plans for dark mode for all Stack Exchange sites
There is already the correct answer that there is currently no native Dark Mode offered by SE for the Mathematics SE (or indeed any SE site). While we wait, I'll just point out that there are browser plugins for e.g. Chrome that allow you to have a (less than perfect) dark mode on pretty much any website. An example screenshot with MathJax: