Have you ever received an esthetic corporate e-mail, something like this one:
Right after that, you get a reply-all follow-up, but it's barely readable due to the new default background color of gray?
Now, if you hit reply, yours wouldn't look much better either. Here is how you can fix it:
Go to the Options tab (1) in your reply window, then select Page color (2), No Color (3). There, wasn't soo hard now was it? ;)