I don't know why, but when I turned this game on, the mouse decided to light up.
Is that a problem with the mouse, or something the game did to override it?

Yeah, that was a surprise for me too, suddenly learning Dead Cells is compatible with the Razer Chroma system.
You can disable that from your Razer Synapse app, either for all games, or a game-by-game basis, Dead Cells obviously being "deadcells.exe."

Maybe I missed it in the Razer client, but where do you change those settings? (Particularly the "all games", since I just want to keep my mouse off).

It doesn't touch my keyboard's settings, which is odd since I have both mouse and keyb razers.

Thanks for the reply