The site is built using Jekyll, and the CSS is Sassified.
Depending on your version of Git, you may need to run these commands to bring in the GOV.UK Frontend Toolkit:
$ git submodule init
$ git submodule update
This should import Sass files from the GOV.UK Frontend Toolkit repository into /transformation/assets/toolkit/.
Next, install dependencies using bundler::
$ bundle install
To start the app::
$ bundle exec jekyll --server --auto
This will compile the site and make it available on http://localhost:4000/transformation. Changes will be reloaded without a restart.