Please check the Editor.EditorBodyStyle and EditorWysiwygModeCss property.
Editor.EditorWysiwygModeCss Property
Specifies the location of the style sheet that will be used by the editable area. Multiple Style Sheets are supported. Example EditorWysiwygModeCss="example.css,~/portal.css,/default.css"
Editor.EditorBodyStyle Property
The style to be applied to the Editor body.
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