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 Gradle. This guarantees that everything is up to date and that packaging is consistent. Gradle is the best build tool that I've come across thus far and I hope and suggest that everyone look into it, as it is the future for JVM languages, if not others. Please note that I have also checked in the "Gradle Wrapper", which allows you to execute the gradle build without needing to install Gradle (though the JVM is required). To do this, just go to the root of the project and execute the command "gradle". The zip file will be created and placed in the "/build" 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://missdawnsdaycare.justinleegrant.com/.