To change the background color of the RadEditor:

1. Create a text file named editor.css
2. Put the following css code inside:

body {background-color:#fff; background-image:none;}

3. Substitute your desired color instead of #fff, which is white.
4. Save the file
5. Upload to the portal root folder using the file manager.

You can also put styles in this file that you want your users to have. They will get loaded into the editor. Remember to copy the same styles into your site.css or stylesheet editor.