How to customize Magento 2 Maintenance mode page?

Edit the file pub/errors/default/503.phtml and put the following code

<?php
/**
 * How to edit and customize code of Magento 2 Maintenance 503 Page
 * Add analytics if any
 * Replace <COMPANY NAME> with your Organization Name
 */
?>
<!doctype html>
<title>Site Maintenance</title>
<style>
  body { text-align: center; padding: 150px; }
  h1 { font-size: 50px; }
  body { font: 20px Helvetica, sans-serif; color: #333; }
  article { display: block; text-align: left; width: 650px; margin: 0 auto; }
  a { color: #dc8100; text-decoration: none; }
  a:hover { color: #333; text-decoration: none; }
</style>

<article>
    <h1>We&rsquo;ll be back soon!</h1>
    <div>
        <p>Sorry for the inconvenience but we&rsquo;re performing some maintenance at the moment. If you need to you can always <a href="mailto:abcd@COMPANY NAME">contact us</a>, otherwise we&rsquo;ll be back online shortly!</p>
        <p>&mdash; Team COMPANY NAME</p>
    </div>
</article>