How is this yellow box called ? How do I edit/remove the highlight effect?

You can remove the highlight effect by opening the Advanced Designer.

1. Go to the toolbar and click on Design

2. Click on Advanced Designer

Go to the "Color Scheme" section and click on the settings of the highlight effect:

Then click on the current color and on the "X" icon, this will remove the color and will set it to transparent, click on select and that's it!

You may also paste this CSS code in the Designer:

.form-line-error, .form-line-active{

    background: none !important;

}