there are two options:

1. enter "Show custom Editor Options" beneath the editor, switcht to portal mode, enter Editor Area CSS section and switch to static. this will affect display of text (font, size) and paragraphs as well.
2. preferred option is to modify the skin of the site and not to apply dark background unconditionally to body tag, e.g. by assigning an ID to the body and use it in your CSS definition.

What you also could do is to change the Backround color only for your Editor.

All Styles for the FCK Editor are stored within the fckstyles.xml file and can be found at this location: ..\Providers\HtmlEditorProviders\Fck\FCKeditor