The various "dark mode" browser extensions work well here. Brave on Android also has a dark mode toggle built in, which I assume repackages one of the extensions.

I keep having to explain that if you use a mobile app with an embedded browser none of those will work.

You're reading HN from an app? Why?

Because reading the RSS feed is much more productive, and I use Reeder Classic on iOS, macOS and iPadOS

Maybe you should look for an RSS reader with dark mode then? I thought they all had that by now.

Spoiler—not even the Android ones do that when they open a link inline.