Background of CKEditor is black for authenticated user
Do you have Firebug installed (extension for Firefox) ?
With Firebug you can inspect element and see what css definition makes it black.
Then, in your theme css file you can override that settings by writting your own definition to make it white. like

.ckebackground {
background-color:#ffffff;
}


where ckebackground is "problematic" class that makes it black.