I set '~/App_Themes/Default/Information.css,~/webadmin/editorStyles/designtimeonly.css' to EditorWysiwygModeCss.
The first CSS is shared between front-end and back-end (CMS) part of the website. The other is just for design-time (which contains only body { } to set the style for editable area).
When CuteEditor shows up it recognizes only the first file. This was proved by changing the files order in the property.
Thanks,
Alex