Edit WooCommerce emails CSS

Paste the following code in your active theme’s functions.php:

add_filter( 'woocommerce_email_styles', 'patricks_woocommerce_email_styles' );
function patricks_woocommerce_email_styles( $css ) {
	$css .= "#template_header { background-color: #231f20; }";
	return $css;
}

Edit CSS as needed.

Source: https://www.speakinginbytes.com/2016/05/customizing-woocommerce-email-styles/