Add CSS to WooCommerce Emails

To change the styling of emails that WooCommerce sends, you can add inline CSS using the woocommerce_email_header hook. Add the following code to your functions.php file:

<?php
/**
 * WooCommerce
 * Add inline CSS to emails sent out
 */
function sv_add_css_to_woo_email() {
    echo '<style type="text/css">
             h1 {
                 text-align: center !important;
                 color: #DDD;
             }
         </style></pre>'; 
}
add_action( 'woocommerce_email_header', 'sv_add_css_to_woo_email' );

For more WooCommerce email customization, see Mastering WooCommerce Hooks.