Quick tip: JCE is a wonderful editor for Joomla! - free, useful and powerful.  But after a default installation (particularly with the useful K2 CCK for Joomla!) we often see the following error message:

* The file name 'templates/templatename/css/editor_content.css' was entered in the JCE Custom CSS field. This file could not be found in the default templates folder. *The css file '/templates/system/css/editor.css' is being used instead.

Here's the fix: