This reply has been accepted as the best answer.
This issue will be fixed in Weaver Xtreme V5.0.4 in a few days.
It results from stopping support for the obsolete WP default background color option which is not really being supported by newer themes.
Turns out that rule scrambler described was part of doing that, but had other unintended consequences. You should be able to add a custom CSS rule with the background color you want if you also add !important to it.