There are two wins in this. 1) Firefox. Which means you also get the extension on mobile. 2) Dark Reader works on all websites (and you can turn it off for sites don't play well, which isn't many).
Here is a Tampermonkey¹ script I've used for some ime to make HN look the way I want it to (plain white background). It should be easy to adapt for a dark preference. It also adds a "bookmark" at the top of each page which is also easily removed if you don't want that.
I use a custom config on Dark Reader extension in all my devices. Having the same color scheme in all webpages are much better UX than what any web can implement by themselves.
If you use Firefox, have my userContent.css DIY dark mode. Or you can massage it into the syntax expected by { Arc’s custom CSS, uBlock Origin, your CSS injecter of choice }:
Obvious caveats apply: it’s mine, it’s certainly incomplete, yadayada. But hey, been working for me for years! Also, now that you know that the feature exists, you can probably search "Hacker News dark mode userContent.css" and find better / more complete ones :)
I added thumbs up emoji for voting, book emoji for read comments, larger user names, larger headers, and dark color scheme.
https://postimg.cc/f3nmR6DG
https://pastebin.com/zeXGi4WQ