How can I change the background color of the fields when a user does not fill out a required field?

I want to change the background color of the "required" text fields as well as the red rectangle that comes up with the "This field is required." message. How can I do that? Please see attached screenshot by RED arrows.

Thank you.

If I understand you correctly, you want to change the color of the background of the field if the user leave it blank. See the image below:

If yes, please add this custom CSS code:

.form-line-error { 

    background: red !important;

} 

Here's a guide on: How-to-Inject-Custom-CSS-Codes.

Result:

Hope this answers your question.