This page provides the links to modules/themes/feathers.
Installation involves downloading the zip archive, and extracting its contents to a similarly-named directory under themes/. You’d then need to activate it via the admin panel.
By the by, I was rather disappointed that the archives were only available as .zip archives. I’d much rather these archives be available in .tar.gz, or some other commonly used *nix format. To add insult to injury, several of the themes - gull and grackle - had .zip archives that produced errors when uncompressed with 7zip in Windows. These exact same archives extracted fine under OpenBSD.