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:
// set the dark background here
// 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.