This is a bug I thought I fixed a while ago (I'll get to it eventually) where if you don't click on settings for the screen saver, pick a value for number of bunnies AND hit "save", the screensaver starts with a value of 0 for number of bunnies. Doing that should fix it for now.
It's sort of on the roadmap! No idea what the pacing for this silly project is going to be like, but I do have plans to make the sprites moddable (maybe drop in sprite sheets and some script files?) and although it wasn't part of the Design Document I was definitely already thinking about how OBS can do chroma keying, and how that'd make it pretty easy to do something where you could add bunnies to a stream...