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";.

Romero’s Night of the Living Dead is in the public domain as well.

You can find it in the Internet Archive here: https://archive.org/details/night_of_the_living_dead

It could be an issue with the codecs (browsers are usually pretty limited in what they support). You could try to use a client like Jellyfin Media Player instead. It bundles libmpv, so it plays almost any video format there is.