Custom CSS for categories?

Great idea!

Add this to your theme’s functions.php or create an mu-plugin.

/**
 * Add category to testimonial CSS classes.
 *
 * For Strong Testimonials.
 */
function my_testimonial_class( $classes, $args ) {
	global $post;

	$class_list = explode( ' ', $classes );

	$categories = wp_get_object_terms( $post->ID, 'wpm-testimonial-category' );

	if ( ! empty( $categories) && ! is_wp_error( $categories ) ) {
		foreach ( $categories as $category ) {

			// To add the ID:
			$class_list[] = 'category-' . $category->term_id;

			// To add the slug:
			$class_list[] = 'category-' . $category->slug;

		}
	}

	$classes = implode( ' ', array_unique( $class_list ) );

	return $classes;
}
add_filter( 'wpmtst_post_class', 'my_testimonial_class', 10, 2 );

Example CSS:

.strong-view .testimonial.category-1 {
  border: 1px solid red;
}

or

.strong-view .testimonial.category-my-product {
  border: 1px solid red;
}

You may need to increase the specificity by adding the template name:

.strong-view.default .testimonial.category-2 {
  border: 1px solid red;
}