my favorite side theme is the chapter 6 episode titles
Looks like someone has an idea of what the side theme is about ;)
But you missed the mark a little bit... the side themes are taken from the recent Winter 2020 anime adaptation's episode titles, not the main game it is based on.
Also just in case... "Rebellion" is not a theme.
"my name" was such a good episode ;w;
To further clarify, I will just reveal one of the hidden side themes:
Episode 1: やぁやぁ、知ってる？魔法少女のその噂 (Have you heard? The rumours about the Magical Girls)
Episode names from the game are not part of the 13 hidden side themes
