It is in the widgets.php file
located at
wp-content/plugins/bbpress/includes/common/widgets.php
start at line 1064
you should be able to find the time in there somewhere
NOTE : this file will be overwritten by bbPress updates. You might do better to fork the code into your own plugin to prevent overwrites.
I think (but am not sure) the styling comes from other wordpres & bbPress elements and has no direct bbpress.css elements