After using them to implement a dark mode (and a pure black variant for OLED), plugging everything into my existing HN browser extension which already lets you apply custom CSS, and making it handle theme switching via attributes on <html>, custom styles and theming are now manageable using it. There are a few examples in the friendly release notes above, with screenshots and copy-pasteable CSS.
If you just want to grab the stylesheet which sets up the CSS variables and application rules to use in your own thing, it's here [1], but not _everything_ is themeable without a bit more work - in particular, I had to replace the HN <img src="y18.svg"> logo with an inline version of the SVG, so its fills can be controlled by CSS.
[1] https://github.com/insin/comments-owl-for-hacker-news/blob/m...