I started looking at this as I'd like to add one to a few of my sites, but soon started down a rabbit hole:

1. Many code examples just handled light/dark and once clicked you cant easily get back to the system setting.

2. Seems folks going about this many different ways.

3. The Flash of Unstyled Content issue.

4. Im a big tailwind fan, but I think a clean implementation should exist just with the most vanilla CSS/HTML and very light use of JS as necessary.

5. Storing the preference once set

6. Discussions on adding this "simple" (seeming) feature bought out a wall of comments when someone suggested adding it right here on HN, and as of today, HN itself still doesnt have this "simple" feature.