The recommended way to modify a theme with Wordpress is to leave the original 
theme, create a child theme, and work with the child.  That's easy enough to
do while working locally, but can be a pain when you're working on a remote
site (which is most of the time.)

Child Theme Assistant is a plugin to help with the creation and management of
child themes in the Wordpress Admin interface.

Requires php version >= 5.0.0 and Wordpress >= 3.0.