The backgrounds are not free. Just copyright-free, meaning that I pay the artist, and I am licensed to use his work commercially. It's almost like commissioning him for custom art, just that other people can use the same work. It's like buying an asset pack from the Unity store or something. It's a pretty common thing. Being this is the first time I would ever attempt to make money, I don't have the largest budget, or pool of resources (connections to artists.) I would like to hire an artist to make original art, but I can't find a quality artist who would work with a nobody like me for an affordable price.
Also, yes, the backgrounds I found do match my game. I'm not making any concessions on that front. I looked at it the way a director would scout film locations for a movie, and made sure to find what would fit.