Add custom CSS to WP HTML Mail WordPress E-Mails

functions.php

/**
 * Add custom CSS to WP HTML Mail WordPress E-Mails
 */
add_filter( 'haet_mail_modify_styled_mail', function( $email_html ){
    $custom_css = ' 
            .container-padding{
                padding-left: 60px; 
                padding-right: 0;
            }

            h1{
                border-bottom: 2px solid black;
            }
        ';

    $custom_mobile_css = ' 
            .container-padding{
                padding-left: 10px; 
                padding-right: 10px;
            }
        ';

    $email_html = str_replace( '/**** ADD CSS HERE ****/', $custom_css . '/**** ADD CSS HERE ****/', $email_html );
    $email_html = str_replace( '/**** ADD MOBILE CSS HERE ****/', $custom_mobile_css . '/**** ADD MOBILE CSS HERE ****/', $email_html );
    return $email_html;
});

Im confused. In the plugin FAQ it says

You can add your own CSS code for desktop and mobile. Just add this example to your (child-)themes functions.php and customize it:

add_filter( 'haet_mail_css_desktop', function( $css ){ $css .= ' h1{ border-bottom: 2px solid green; } '; return $css; });

However, this does not work and there is no haet_mail_css_desktop defined in the plugin.