Add custom CSS to HTML emails

Append CSS to the styles used by WooCommerce and AutomateWoo HTML emails. Test the result in multiple email clients because CSS support varies.

Developer note:

Add custom PHP in a small custom plugin or child theme, and test it on a staging site before production. This example requires ongoing maintenance when extension APIs change.

functions.php

<?php

add_filter( 'woocommerce_email_styles', 'my_store_woocommerce_email_styles' );

/**
 * Add styles to WooCommerce and AutomateWoo emails.
 *
 * @param string $css Existing email CSS.
 * @return string
 */
function my_store_woocommerce_email_styles( $css ) {
	$css .= '#template_header { background-color: #7532e4; }';

	return $css;
}