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";.