If you theme yields: No reference to
add_editor_style() was found in the theme. It is recommended you implement editor styling.
This should make the editor content match the resulting content, for a better user experience.
To do that you need to use CSS styles from the frontend to the backend editor.
You should create a file like editor_style.css that is a subset from the main web site styles. This file will contain styles directly related to the post content such as: typography, links, images, galleries,…
Finally you need to call
add_editor_style( array("css/editor_style.css") ) to inject the styles.