HN's extremely poor usability, accessibility, and design are topics covered at length for years and years now. The catalog is filled with them. Search hn.algolia.com and you'll see.
Dunno if it's perfect but ironically, the minimalism is really nice compared to reddit's. New Reddit is obnoxious and Old Reddit's various elements eventually collapse on each other and make it an unreadable mess.
Well it depends strongly on the person. I'm sure many people would hate Hacker News' layout. But for me it is perfect. In fact I've built some custom CSS to replicate the same for other sites as well.
In your search, I'm sure you'll find that in every thread where such a claim is made, many of us reply that HN's design is excellent - a real breath of fresh air, compared to most of the modern web.
For dark mode I just use dark reader.