When you use the Smart Tweak in Toolbar Extras called “Use Admin Color Scheme also for Frontend” then it could happen in some edge cases that there is a conflict with your body background color on the frontend.
The reason for that is the “admin” stylesheet that gets loaded by this tweak and unfortunately includes styling for the body background. With some themes this leads to overriding the color set by the theme.
If this happens to you just do not use the tweak, set it to “No”.
I am working on a global fix for that so that theme conflicts will no longer be an issue. However, this is a rather complex topic as a lot of aspects to into it and I want to do it the right way. So it could take a bit of time since this will be solved forever. Thanks!