add_action( 'wp_head', function () {
echo '<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.2.0/styles/default.min.css">';
});

add_action( 'wp_footer', function () {
echo '<script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.2.0/highlight.min.js"></script><script>hljs.highlightAll();</script>';
});