Unpossible! I have been informed that Plex and Jellyfin are the same essentially.
There is theme song support, but you have to add them as local theme.mp3 files. They are supported on the web interface, media player PC app, and the mobile client. I don’t believe they are supported on any client with a 10-foot (smart TV) interface.
Movie trailers are done as links that open externally in YouTube if you don’t have a trailer file locally in your library. This works but it really a non-solution on the TV interface I found since it requires leaving the Jellyfin app.
The option to use them is under the Developer Options on the Android TV client’s interface. They do work from the limited testing I did (I don’t enable the feature generally because of the space images like that take up on storage, same on Plex).
