The Lead Topic ( content-single-topic-lead.php ) is a very powerful but super simple function that can be used to style the Topic Authors different from the Repliers..
It’s also very useful if you want to add some ads or a simple note to your readers after the first message ( from the topic author)
Bummer this is nowhere to be found in the Codex.
But you can activate this option by adding this line of code to your WordPress theme functions.php template file.
Note i say functions.php and not bbpress-functions.php because you if you would add it to the bbpress-functions.php this code will be overwritten when the bbPress plugin updates in the future.