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

add-editor-style.php

<?php 

function wds_add_editor_style( $stylesheet = 'editor-style.min.css' ) {

	if ( is_admin() ) {
		// Add styles to the post editor
		return add_editor_style( $stylesheet );
	}

	/*
	 * We're doing this for CMB2 front-end wysiwyg
	 */
	global $editor_styles;
	$editor_styles = array_merge( (array) $editor_styles, (array) $stylesheet );
}