I agree with many of the other similar comments, having dark-mode supported by default would be good BUT we don't need it. It's already well supported by several styling extensions.
I use 'Stylish' on Firefox and it works well not just on HN but quite a few sites.
https://userstyles.org/styles/22794/a-dark-hacker-news
Here's what my HN looks like - https://imgur.com/a/UM5qXtD