[Adam Harley (Kawauso)]

Easy enough to do

<?php if (40 <= $topic->topic_posts) : ?>I'M ON FIRE<?php endif; ?>

Put that in your front-page.php and/or forums.php somewhere inside the parts that say foreach( $something as $topic ), probably best before <?php bb_topic_labels(); ?>. Change 40 to whatever you want the topic to change at and the on fire bit to something… fiery.