BuddyPress.org

You want to edit forums/forums-loop.php

Find the <tr> under

<?php while ( bp_forum_topics() ) : bp_the_forum_topic(); ?>

and add

<tr class="<?php bp_the_topic_css_class() ?><?php if(bp_get_the_topic_poster_id() == bp_loggedin_user_id()){echo '-topic-author topic-author';} ?>">