I'm pretty sure there must be some background color applied directly to body{} in one of the css files in your skin.
you could explicitly set
body {background-color:white, color:black;}
in one of the css files, ideally one lower in the list in style.config so it comes in after anything else that might be setting the color.
then put the color back on the page with
body.pagebody{}
Note also, that in the next release of mojoPortal (coming next week), I have implemented a special class on the body for the editors so it will be easier to override styles in the editor like this:
body.wyswygeditor { background-color:white; color:black;}
Hope it helps,
Joe