Hello,
I am new to this game and I wondered: Where do you get backgrounds such as "Graveyard"? It seems to be neither basic nor premium. Do you only get it if you make a custom wolf?

Some background come from events. I imagine that one came from a Halloween event.

Certain ones can be won if there is a site event going on, which usually happens during holidays. There is a currency you collect while exploring during the event, and reaching a certain amount unlocks that prize.

This guide may help

Thank you all for the responses, that helped me out a lot. :)