To modify it for yourself, you can open the Dark Reader flyout and click Settings > Advanced > Dev Tools > {your preferred theme} Editor > Per Site Editor. There you can add any CSS tweaks you like. To my knowledge there isn't a community-maintained repository of theme mods, but if your modifications fix something that's currently broken in Dark Reader's default theme, then you could fork the project and submit a Pull Request for probably one of these files: https://github.com/darkreader/darkreader/tree/main/src/config (depending on which theme you're editing).

If your modifications aren't fixing a problem, and they're just tweaking the dark mode to better fit your preference, it will probably not be accepted, though.