Add A Horizontal Rule (HR) Button to the WordPress Editor

functions.php

<?php
//* Add HR button to TinyMCE
//* @link http://anythinggraphic.net/horizontal-rule-button-in-wordpress

add_filter("mce_buttons", "ag_horizontal_rule_button");
function ag_horizontal_rule_button($buttons) {
     $buttons[] = 'hr';
     return $buttons;
}

style.css

hr {
	border-top: 1px solid #ccd8db;
	border-bottom: 1px solid white;
	clear: both;
	margin: 3rem 0;
	width: 100%;
}