It’s a one-off theme, made open source as an example of how you can customise a Ghost site via the theming system, it’s not designed to be a generic re-usable theme.
From the readme:
The theme for our official company blog. This is fairly heavily modified version of the Casper theme, with a bunch of hard coded variables. It will not automatically work on any Ghost installation.
Also note that https://ghost.org/blog/ is no longer using this theme, although the design is mostly re-used. The company blog is currently built using gatsby as part of the main ghost.org site but it still uses Ghost as the content source.