Can't quite remember how it looked when I posted last, but I certainly couldn't see a white background then in either Firefox or IE8.
Now I see a black background showing in both embedded and stand-alone forms. When embedded, the hard edge of the black background just intrudes onto the lighter menu area.
In other words it sounds as if it is displaying as expected given your CSS you say you have in place.
I wonder if the fix Hugo applied is still not working with your particular browser. What OS and browser, including versions are you using. That info may help Hugo.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.