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"    

Editor.EditorBodyStyle Property:

The style to be applied to the Editor body.

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

Hope it helps.

---

asp.net Chat http://cutesoft.net/ASP.NET+Chat/default.aspx
Web Messenger: http://cutesoft.net/Web-Messenger/default.aspx
asp.net wysiwyg editor: http://cutesoft.net/ASP.NET+WYSIWYG+Editor/default.aspx
asp wysiwyg html editor: http://cutesoft.net/ASP
asp.net Image Gallery: http://cutesoft.net/ASP.NET+Image+Gallery/default.aspx
Live Support: http://cutesoft.net/live-support/default.aspx