There have been several instances of feedback asking for a "dark version" (dark background, light text). I don't know if vector tires the eyesight more than monobook or it's simply that they now have a way to ask for it, but the petition was higher than I expected.
Probably more appropiate to provide as an alternate stylesheet than a separate skin, though. I wouldn't be surprised if some wikipedian already designed it.
WMF-developed dark-mode userscript usable since October 2019, showing complementer colors with the css rule transform: invert(1)and specific cases fine-tuned for legibility.
Dark-Mode project to implement this in MediaWiki.
T122924: Merge Extension:Theme into core
Skin themes – dark and gray themes for Timeless and Vector skins. Custom color palette, similar to discord's colors. Experimental volunteer project: there is some content that's hard to read with it.