According to this post on MSO:
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 [styles] 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.
It might be a long time before a dark theme comes out for EL&U.
See also: Plans for dark mode for all Stack Exchange sites
For the time being, you can try out (third-party) workarounds.
If you really want to view EL&U in dark mode, you can try out Dark Reader. This is how it would look like on
See also: Post by canon on MSE
Caveat: Toast notifications will appear blank when in Dynamic mode (you can switch to Filter mode, although it's not as great). Also note that the dark theme will be applied to every website on your browser when you've installed it, but you can toggle it off for that website individually (or you can apply this change only to sites you want to apply it to).
It's not perfect, but it should work for the time being.