Sorry to come back to this.
Have a look at this demo page
on your site. Now, you're prolly not looking at it with the black skin, so way don't you go back and look at it again, this time changing the skin to Black. To avoid ambiguity, I've attached a screenshot of who that page appears in IE8 at this end.
I accepted the argument that you'd forced the background to white for the reasons your stated, but the whole thing falls down if the only
bit of the theme you've overridden is the textbox Background colour!
believe that you should leave well alone; if a user has selected skin, they will have done that in the knowledge of what it entails. Having you move the goalposts (or, as it appears - just one of them) on the quiet isn't helping.
At the very least
you need to fix the Black skin for the RadEditor.