Please use the following property:

Editor.EditorWysiwygModeCss Property

 

Editor.EditorWysiwygModeCss Property allows you specify the location of the style sheet that will be used by the editable area. Multiple Style Sheets are supported.

Code Example:

<CE:Editor id="Editor1" runat="server" EditorWysiwygModeCss ="~/example.css"></CE:Editor>

 

Demo:

 

 

Regards,

 

Ken