chess.com are you serious fr bruh?

I don't know if the option moved, or if we both forgot where the option originally was, BUT:
You can upload your custom backgrounds under All Settings => Interface => Aaallllll the way at the bottom, under the Beta Testing even. Changing the setting under Boards and Pieces => Background -will- override it, but that's not the same place you upload custom backgrounds. The two options are well separated in the interface...for reasons that probably make sense to somebody, but not to me.
I wanted to change my background, and when I did, there was no custom button upload button due to a series of updates on appearance.