Please check EditorWysiwygModeCss property and EditorBodyStyle :

EditorWysiwygModeCss:
 
Specifies the location of the style sheet that will be used by the editable area. Multiple Style Sheets are supported.

Example:   editor.EditorWysiwygModeCss = "example.css,~/portal.css,/default.css"    

For example, you can add the following code into your style sheet file: 

Editor.EditorBodyStyle Property:

The style to be applied to the Editor body.

Example: editor.EditorBodyStyle="background-color:tan;Color:white; font-family: Verdana ;"

Hope it helps.

dudu:

Only Editor.EditorBodyStyle  is effective.

 

Please check this example:

 

http://cutesoft.net/example/Using-External-Style-Sheets.aspx

 

It uses EditorWysiwygModeCss property.

 

 

Sorry! Now it is ok.

Because of the client cache.