This is a Jekyll + Github pages site. Pushing to the repository on the master branch compiles the site and pushes it live. If developing non-live changes, don't push to master!
The site is built upon Jekyll, a static site generator. Install Jekyll via ruby gem. There's a good installation guide here. You might have to update ruby or several other dependencies as well.
To test locally, start the server. Run jekyll --auto --server at the root directory of the codebase. The site will be hosted on http://localhost:4000/ (there's a chance your port could be different).
Jekyll will now compile the site any time a change is made to a file in this directory. These changes compile static files that can be found in the _site directory.
To push changes live, simply push the master. Github automatically compiles the site and makes the changes live!
The design for the site was built using SASS, a markup language that compiles down to CSS. It's really awesome. Check out the documentation for details on how to install and compile the SCSS files.