The demo code is publicly available on GitHub. You are welcome to fork that project.
Our recommendation is to use the demo as a reference and bring over things that you want into a project based on the Archetype structure. This will be more compatible with future Broadleaf upgrades and support a more maintainable programming model as your needs grow.
We've written some documentation to help you understand how to customize Broadleaf. You can find it on our documentation site, located here: http://www.broadleafcommerce.org/confluence/display/BLC15/Customizing+Functionality