I believe that the background of the text editor inherits the background color of the skin, which is why you're having that problem.
You can override nearly any CSS using the Portal.css file, which is directly editable at the bottom of the Site Settings. Try adding the following CSS to your Portal.css file and see if it helps you.
.reContentCell, .reContentCell iframe, .reContentCell * { background-color:#ffffff; }
However, due to how and when telerik imports their skin style for the text editors, you might have to end up putting that CSS in the override CSS file in the following directory:
~/Providers/HtmlEditorProviders/Telerik/EditorOverride.css
Finally, you could also use a bit of jQuery to force this to happen.
jQuery(document).ready(function(){ jQuery('.reContentCell, .reContentCell iframe').css('background-color', '#ffffff'); });