Very simple thing yet I've spent almost 20 minutes trying to figure it out and find how to do it.

The thing is that ckeditor uses white background for editing area and the sites background is dark, so text color was set to white by the client and when editing in cme nothing could be seen.

In order to feed ckeditor a stylesheet with all the style definitions that will be used in the editor area just use this:

CKEDITOR.config.contentsCss = "/css/admin/editor.css";