Brumby,

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.