Generally the editor is configured to have the same css as the site so that the user will see things in the editor as they will look outside the editor.
There are occasions when you may have a background color or image on the site but you don't want that background color in the editor. The way to set a background color or image on the site without having it in the editor is to not put it directly on the body like this:
body{ color: #666666; background-color: #FFFFFF;}
but instead add it to a class selector like this:
.pagebody { color: #666666; background-color: #FFFFFF; }
then in your layout.master file set that class on the body like this:
<body class="pagebody">
Since the page shown inside the editor won't have this class on the bod it will not show the same background styles as the site.
Hope it helps,
Joe