Sorry to hear about the problem you’ve run into. Puro Free doesn’t have a background option under Appearance > Customize but Puro Premium does. If you’re running Puro Premium please go to Appearance > Customize > Colors > Background Color and make the change there.
If you’re running Puro Free, let me know and I’ll work up a little Custom CSS that can be added to a plugin like Simple Custom CSS to achieve the same outcome.
I think you might have reached out to me on email too. I thought your question above applied to our first theme, also called Puro. You’re running Ultra. The answer would be the same. We unfortunately don’t have Ultra Premium out just yet.
I think let’s just move this thread over to email and I’ll help out there.