the html structure of the testimonial section is not structured to allow for an easy solution;
example structure from the existing testimonials:
<div class="DisplayItem">Dear Mister</div>
<p>We are very pleased with the result,
ramble ramble ....
... and compliment a great product.</p>
<p>With best wishes.</p>
<p><strong>Mr & Mrs Jones (City)</strong></p>
ideally, this section would be wrapped in a html tag with its own css class;
example:
<span class="testimonial">
<div class="DisplayItem">Dear Mister</div>
<p>We are very pleased with the result,
ramble ramble ....
... and compliment a great product.</p>
<p>With best wishes.</p>
<p><strong>Mr & Mrs Jones (City)</strong></p>
</span>
and add a new style to style.css of your theme:
span.testimonial { display: block; border-bottom: 1px solid #ccc; }
[duplicate thread: https://www.ads-software.com/support/topic/how-do-i-add-in-a-div-line?replies=3; please continue with the other thread; this one is closed per forum rules]