I normally use LESS files in Drupal instead of CSS for easier stylesheet maintenance. However, it looks like specifying a LESS file as CKEditor custom css file does not work. The custom "Editor CSS" file which is specified in CKEditor profile->CSS tab, must have ".css" extension to work (otherwise it is loaded literally as a .less file which the browser does not understand). It would be good if the stylesheet were loaded in such a way that it would be preprocessed by the LESS CSS Preprocessor, the same as other stylesheets are preprocessed. Otherwise, to have the WYSIWYG effect, the stylesheet for nodes elements must be redefined using normal CSS coding.