add editor style (works for front-end editors as well)

add-editor-style.php

<?php 

add_filter( 'mce_css', 'jpry_add_editor_styles' );
/**
 * Filter the TinyMCE styles.
 *
 * @param string $styles The comma-separated list of stylesheets to be loaded by TinyMCE.
 *
 * @return string The updated comma-separated list of stylesheets to be loaded by TinyMCE.
 */
function jpry_add_editor_styles( $styles ) {
	$new_styles = array(
		get_stylesheet_directory_uri() . '/editor-style.css',
	);
	
	// Add our styles to the array
	$styles_array = array_merge( explode( ',', $styles ), $new_styles );
	$styles       = implode( ',', $styles_array );

	return $styles;
}