How long have you used it and how is it?

I’m pretty curious about those kinds of distros, and don’t really like how nixos is completely hosted on github (and all the drama that constantly comes from the community, and the bad documentation for many things, …).

However, guix seems such a niche project that I feel like it can’t really be used.

It’s a path inside the container, but not inside /config. You should mount the file like this:

volumes:
  - /path/to/local/theme.css:/jellyfin/jellyfin-web/theme.css

You should put it in Jellyfin web’s root folder (paths are relative to it). In the official container it’s /jellyfin/jellyfin-web.

Then you just @import "theme.css";.

It is not different from how the previous shared libraries worked. I guess it’s there to stop cheaters from buying a single copy of the game and sharing it with throwaway accounts.