Hi Chris,
I know a quick and dirty trick for that one:
1 - Get this file /DesktopModules/Admin/RadEditorProvider/Css/EditorContentAreaOverride.css
2 - Modify the background-color of the Body;
3 - Clear you local cache;
There might be a better way to do that, that is not such a hack, but I'm not aware.
This should work. It does for me.
Cheers,
Aderson