Highlight comments of all the site authors

There’s a filter called comment_class for that :

apply_filters( 'comment_class', $classes, $class, $comment->comment_ID, $comment, $post_id );

Source

Just use the argument comment which is an object and you’ll get $comment->user_id which could be useful to get user role and add your classes.

EDIT:

add_filter( 'comment_class', 'wpse_253517_comment_class', 10, 5 );
function wpse_253517_comment_class( $classes, $class, $comment_ID, $comment, $post_id  ) {

    if ( 0 === (int) $comment->user_id  ) {
        return $classes;
    }

     $user_data = get_userdata( $comment->user_id );
     $role      = reset( $user_data->roles );

    switch( $role ) {
        case 'subscriber':
            $classes[] = 'is-subscriber';
        break;

        // here would be other cases, other roles     

        default:
        return $classes;
    }

    return $classes;

}