Hello!
We're assuming that you are referring to the theme of the zine itself. Here, "muscial themes" can be almost everything that relates to music.
It can be characters making/experiencing music in a canon environment, it can be a music related AU such as idols or band AU, it can be a character study what a character associates with music, a piece inspired by lyrics or something completely different, as long as there is a connection/reference to music somewhere in the piece.
We hope this answers your question and thank you for bringing it to our attention, that this wasn't explained in our carrd so far - we will make sure to add that information.
If you'd like to know more, feel free to ask again :)