Display Borders

Only the default template has a border. I can provide some custom CSS to add a border to the other templates if you want. Which template are you using?

For the Modern template, for example:

.strong-view.modern .testimonial {
  border: 1px solid #F0F0F0;
}

The template names are listed here:
https://strongplugins.com/document/strong-testimonials/custom-css/html-elements-css-class-names/

Where to put custom CSS?
https://strongplugins.com/document/strong-testimonials/custom-css/where-to-put-custom-css-code/