You can introduce a dark theme for HN without feature flags or CSS upload. Support for `prefers-color-scheme` is widely supported in all the browsers[1]. It's how I handle it on my blog[2], I just swap out the colors based on the css selector.

1: https://developer.mozilla.org/en-US/docs/Web/CSS/@media/pref...

2: https://www.notcheckmark.com