Add checkout validation styles to a third party or custom theme

You need to add this CSS to your child theme’s style.css file or to your customizers’ “Additional CSS” section. Please don’t add custom code directly to your parent theme’s style.css file as this will be wiped entirely when you update the theme.

https://gist.github.com/woogists/f98721aaeee22daed22e1507714b854b