We don't have a global style for tables, so I had to delete sections from our stylesheet until I finally narrowed it down. We had a CSS class for our dialog windows defined so that they would have a very light gradient in the background. This one line is what was causing the browser background and the RadWindow to all go black:
filter:progid:DXImageTransform.Microsoft.Gradient(GradientType=0, StartColorStr='#fffdf4', EndColorStr='#fffaf0');
The weirdest part is that the gradient is from one off-white pastel yellow to another. Black is not even involved. I commented out our default background color of black at the <body> level too, but that made no difference. It was just the gradient style.
We have taken the gradient out. It's not important to us.
Thank you for your help,