Hi Andy
Thanks for running Puro.
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.