As Joe Davis has mentioned, generally you do want the editor to use the skin css so that things look the same in the editor as they will after editing. The techniques he has suggested should be able to resolve those cases where you don't want background colors or images in the editor.
However, I will also add a config setting to allow you to override the css used in the editor for the next release. I just added it to my copy.
The setting will be as follows:
<!-- leave this blank to use the skin css in the editor which is generally the best approach
but if you need to you can specify a different url for css to use in the editor
<add key="EditorCssUrlOverride" value="" />
Of course this is not a site specific setting so it will apply to all sites in an installation if used.