Customizing WooCommerce and AutomateWoo email CSS with a filter

functions.php

<?php

add_filter( 'woocommerce_email_styles', 'my_filter_woocommerce_email_styles' );

/**
 * Filter email styles for WooCommerce and AutomateWoo.
 * 
 * @param string $css
 *
 * @return string
 */
function my_filter_woocommerce_email_styles( $css ) {
	$css .= "#template_header { background-color: #7532e4; } ";
	
	return $css;
}