It's gated behind a CSS "prefers-color-scheme" media query, so it will not affect people like you in any way, who haven't opted-in to dark mode in your operating system.
Some people suffer from things like eye strain and this would improve the accessibility of the website.
>So why go for dark mode if legibility, detail perception and performance is lower?
Ok you're arguing that dark mode itself is useless. I don't know enough about it to have an opinion on it one way or another, I can find sources on the web going either way.
What I'm asking for is the evidence for the slippery slope implied by "gateway" whereby Hacker News staff will be more likely to add further "useless" features simply by having added one. Assuming for the sake of argument that it is useless.
> Ok you're arguing that dark mode itself is useless
I didn't say it's useless; there are use cases where dark mode is useful, e.g. "people with cataract and related disorders may perform better with dark mode" (from the Nielsen study). But it doesn't make much sense to add a feature just because it is fashionable, and not because it has a true advantage. If the people in charge of HN decided to change their site, there would be far more useful features than dark mode. I think that was the OP was referring to with "gateway to more useless stuff": if fashion and not usefulness is the number one priority, then a good product could render useless and development resources are wasted.
Some people suffer from things like eye strain and this would improve the accessibility of the website.