A few small changes

With custom CSS:

/* round image */
.strong-view.default .testimonial-image img {
    border-radius: 50%;
}

/* no border on image */
.strong-view.default .testimonial-image img {
    border: none;
    padding: 0;
}

/* star color */
.strong-rating span.star:before {
    color: red;
}

More examples:
https://strongplugins.com/document/strong-testimonials/custom-css/default-template/

Thanks!

The round images work but they still have a border and the star colour won’t change…

Try clearing your Autoptimize cache and minified CSS cache.