I don't really think that this should be called a theme issue. It has to do with the default settings in PHP-Fusion's initialisation of TinyMCE (in themes/templates/admin_header_mce.php):
That is ok, but it gives you a lot of style-declarations that are useless for news and articles, and if it contains a declaration for body's background, that will be used in the editor.
The most viable solution is to make your own stylesheet and alter the
setting in the initialisation to:
One of your own declarations could be:
Then your editor will be unaffected of any theme's style.