For everyone like me that had no clue how to do this:
In the menu go to Plugin Editor under the Plugins button. Then on the right select the WP Maintenance Mode plugin and in the menu on the right click Includes -> Functions -> Hooks.php
I copied @georgejipa snippet of code including a closing tag ?>
and added it here: wp-maintenance-mode/includes/functions/hooks.php
See how it looks on my website
I used a font from Google Fonts. Here’s my code:
function wpmm_add_custom_css() {
?>
<link href="https://fonts.googleapis.com/css?family=Raleway&display=swap" rel="stylesheet">
<style>
body {
font-family: 'Raleway', sans-serif;
font-size: 38px;
line-height: 1.0;
}
label {
font-family: 'Raleway', sans-serif;
font-size: 18px;
color: white;
}
</style>
<?php
}
add_action('wpmm_head', 'wpmm_add_custom_css');