Just a thought; I'm wondering if it could be possible for -- night owls -- to somehow "inherit" the newly developed "dark mode" on Stackoverflow also on sub-sites, in particular on our, here on GIS.SE?
This is not something that we can do from GIS SE. It would need to come from the SE developers.
Currently, Dark Mode is available as a beta test on Stack Overflow only. More details can be found in this Meta Stack Overflow question. At the moment, it isn't clear if and how/when it will be rolled out to other sites in the network:
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.
Other answers to that question mention extensions you can use with your browser as a workaround to achieve a level of dark mode.