Yep, came here to suggest exactly this. Especially because then it works for all sites, not just HN. I can also second Dark Reader specifically; it's nicely tunable, supports per-site toggles, and lets you set a schedule to automatically toggle dark mode in the evening. I think my only complaint is that it might be the reason why my Firefox on Android gets really slow / hangs occasionally, although that might be unrelated; haven't gotten around to testing (and it might also be the old-ish hardware and >>100 tabs, so grain of salt).

It makes FF on my high-end desktop slow too, sometimes.

In the dark reader popup window you can set the Mode; the default of Dynamic does an excellent job of making everything dark but is pretty slow. The other modes have basically no performance impact but you might see some visual bugs from time to time.

you have more than a 100 tabs on android? what device? how do you even move between?

Yeah, the tab counter hit "∞" (>99 tabs) ages ago, but they're lazy-loaded so it's just a little slower and a lot more scrolling to get to older tabs. Moto G5 Plus (XT1687), 4 GB of RAM, crDroid 6.5 (Android 10) if it matters.