Hello,

The message background is changed by a custom CSS. Add this code to the Theme Settings > Custom CSS > Global

body .woocommerce-message{
background-color:red;
}
body .woocommerce-info{
background-color:red;
}

Replace the “red” with the color as per your needs. If you find a message which has not changed the color, provide page URL and specify what steps I should take to see it. I will provide CSS for it.

Best Regards