HN does not
itself have one, that's the point of the Minimalist mention. It's an extension that lets you override parts of any website. On looking at what is actually active for me on HN, it's actually another extension I use, named Stylus, (which does much of the same thing, but Stylus is focused on just the CSS while Minimalist also lets people provide modules which provides a list of options to turn on/off) which provides the Dark HN theme. I've gone back and forth between a couple of different ones over the years, and one of the older ones was Minimalist, but the one I use now is apparently Stylus -- been a few years since I touched the settings.
Stylus is here: https://chrome.google.com/webstore/detail/stylus/clngdbkpkpe...
You can find themes to suit you here: https://uso.kkx.one/browse/styles?search=Hackernews (or search for any other site you might want to alter)
> Whenever I saw anyone attempting to have everything dark, half the things looked awful, the dark theme was not even an afterthought...
That's exactly why you pretty much need an extension like Stylus or Minimalist to get proper themes made by people who cares what the sites end up looking like.