Can't change the CSS

There's a certain CSS element in this theme that I can't find it's source file. It controls #post-entry and listed along with the other ids (#content, #container #post-entry ).
If you go to my site here http://tabcv2.tabc-us.org/?page_id=29 and check <div id="post-entry"> you will see that it's not in any of the css files.
How can I change this particular style?