This is a good example of a company that has done a good job of designing their Templates for extension. This is an example of a child extension.
In this example, the plugin gets bundled using Maven and the assembly plugin. This guarantees that everything is up to date and that packaging is consistent. You are required to have this installed prior to executing a build. Maven is a Java (primarily) build tool and is a little more complicated than it could be, and I suggest checking out my Sky Child Theme. To execute this, just type "mvn clean install" from the root project directory. The zip file will be created and placed in the "/target" directory.
Theme development is described by reading the article at http://codex.wordpress.org Theme_Development.
Child Theme development is described at http://codex.wordpress.org/Child_Themes. You can also find a simple example that just modifies the css by following the instructions at http://op111.net/53/.
This child theme extension is meant to be an example for others writing child themes and for folks that are extending themes from https://www.elegantthemes.com.
The final product for the website is at http://www.justinleegrant.com.