Yes just had this too...
It only works in the editor_contents.css it won't work in EXTRA.css even with !important
I have had to remove the "body," part of the css as I have a body background colour specified and forcing it to be white for the Editor input box forces the body background to be white too.
Is this going to affect anything else? Removing the "body," part of the following CSS I mean...
Code:
body,
body.mceForceColors {
background-color: #FFFFFF !important;
color: #000000 !important;
}