What is the license?
oops, sorry! i haven't tried itch.io before, so i'll have to look into where to include these things...to answer your question; you can use these for whatever you want :] credit is nice, of course, but it's not required
So is it public domain (CC0)?
yes! :] they're public domain (CC0)
In this case you should add license metadata. Read here how to do it