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;
});

Thanks, that's awesome! Will it make its way into the plugin itself in one of the new updates?

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.