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.

You should put some quotes where you use the array:

not_what_you_think=( "a b" "c" "d" )
for sneaky in "${not_what_you_think[@]}"; do
  echo "This is sneaky: ${sneaky}"
done

This is sneaky: a b
This is sneaky: c
This is sneaky: d