Centering the whole gallery

Just need to know how to center the whole gallery, which file to alter.

Thanks

The following information is required to get an answer:
Get this information from the PHP diagnostic (in the configuration wizard).
Gallery URL http://www.downstreambar.com/gallery/
Gallery version: Gallery v1.4.1-pl1
Apache version: apache 2.0
PHP version (don't just say PHP 4, please):
Graphics Toolkit:
Operating system:
Web browser/version (if applicable):
database: MySQL 3.23.58