I think it is because your theme's styles.css has a setting for BODY and that setting has a declaration for background-color.
Find this line in administration/subheader.php:
content_css:'".THEME."styles.css',Disable ("outcomment"
it. Embracing it with /* and */ will do. Now the background will probably have become light grey, and the selectbox for Styles will be empty.
Now, if you don't need CSS for your articles that will be it. If you do need CSS, you can write your own stylesheet with the BODY setting you perfer for TinyMCE. And the content_css line should be changed into something like
content_css: 'full/path/myown.css'.