I wish very much that this'd happen. It wouldn't be hard, either - HN is known as a very light website and in a test [1] I did to see for myself how the light/dark switch worked it added less than 1K of CSS to my page. I've been using Dark Reader (the browser extension) [2] to change sites to their dark modes, but on older computers like my Aspire One from 2008 it can take close to a minute for a page to be converted to its dark theme, on top of the already-long loading times for heavier sites.
[1]: view-source:https://www.instantfloppy.net/trudeau/
[2]: https://darkreader.org/