Are these not nice looking? Some of the BGs are weaker, that's true, but this game was also quite an early adopter of generated backgrounds, using them when the technology was only just being introduced. I've made a few improvements here and there since then, and I'm hoping it'll only get better moving forward. It's a lot more flexible than just using stock images, or editing images from google
You won't be able to replace it on the most recent version. I really don't see what the big deal is, who sits on the main menu for the entire 6 minutes it takes for the song to play? You can just download the song yourself and listen to it whenever you want.