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.
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