All you need to do is start a new line in your text to get a <br> tag. If you need to start a new paragraph (<p> tag), the you would have to follow @lukasbestle’s advice. You can then style the p-tags within the div if you add a class to it.
For anyone curious about an alternative, I think this CSS selector is more descriptive of the desired effect, as @lukasbestle began to explain, above.
.testimonial__body p {
font-family: serif;
}
This way, you can use the vanilla Kirbytext or Markdown output inside a container with the testimonial__body classname, and let the “cascade” apply the rules to individual paragraphs.
This is effectively the same solution, except that you can quickly add more rules to treat Kirbytext output differently based on the testimonial’s context: