how do I disable the error highlight

If you want to disable the background, like this:

Inject the following CSS code as explained on this guide: http://www.jotform.com/help/117-How-to-Inject-Custom-CSS-Codes 

 .form-line-error{

    background: transparent;

    width: 300px;

}

If you want to remove also the message "This field is required":

Also, include this code:

.form-error-message{

display: none !important;

}

Let us know if you need more help, we will be glad to assist you.