Reply To: add a specific class

create a directory on your theme called ‘bbpress’
ie wp-content/themes/%your-theme-name%/bbpress

where %your-theme-name% is the name of your theme

find
wp-content/plugins/bbpress/templates/default/bbpress/loop-single-forum.php
Make a copy of this file, and put in in the directory called bbpress that you created above, so you end up with
wp-content/themes/%your-theme-name%/bbpress/loop-single-forum.php
bbPress will now use this template instead of the original
and you can amend this

amend line 12 to

<ul id="bbp-forum-<?php bbp_forum_id(); ?>" <?php bbp_forum_class('', $classes[] = 'hello' ); ?>>