It looks like the FCK Editor is importing the following stylesheets for the rich text preview:

• default.css
• skin.css
• portal.css

So, if that background of your FCK Editor has changed, that's probably from your skin defining a background color/image for the body element.  If you change that to only apply to the body of the main DNN page (which has an id of "Body"), I would think it should at least start to address the issue.

So, in your skin.css, change body {...} to body#Body {...}

Hope it helps,