apply_filters ( 'jetpack_relatedposts_filter_post_css_classes', array array(), string $post_id )

Notes

You could use this filter to add each post’s category slugs to the container’s class, like so:

function add_css_classes_on_related_posts( $classes, $post_id ) {
	$categories = get_the_category( $post_id );

	foreach ( $categories as $category ) {
		$classes[] = 'category-' . $category->slug;
	}

	return $classes;
}

add_filter('jetpack_relatedposts_filter_post_css_classes', 'add_css_classes_on_related_posts', 10, 2);

That can be helpful when one wants to style Related Posts differently depending on the post’s category, for example.