Try the following

If you use a stylesheet file,

 

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

 

If you use common.config,

 

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

 

It worked for me.