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/