Each folder has all files needed to apply to the Rivendel template for the design, along with a folder of screenshots. You can navigate these files directly but the best way is to use the builder UI:
New designs are welcome :) Make a pull request with a new folder in either headers or footers (or a new one) for your new design.
Please add screenshots & all files required in the paths as they would be used, then also update the docs/builder.js to show off your new design. Thank you.