Please check the 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"

 

Then add the following code into your css files:

 

 hr
{
 height:1px;
}