If your web site uses a dark body background, you may want to use a different background for the WYSIWYG editor. In such case, you need to specify the CSS styles like these:

.LTR, .RTL
{
  // set the dark background here
  background-color: black;
}

BODY
{
  // do not set any background here ...
}

The LTR/RTL style is used only for the web page background, while the BODY style is used for both the page and the WYSIWYG editor. Alternatively, you can create a separate CSS stylesheet and assign it to the editor in Site Manager -> Sites -> Site Properties -> General dialog.