Chris Best of Substack replied: https://twitter.com/cjgbest/status/1602370307884318720
Substack is not "powered by Ghost". Rather, we built our own theming API that’s compatible with themes built for Ghost, including those built by third parties.
The Free Press is using a modified Tripoli theme, built by Ahmad Ajmi, under a paid license. This is how this is supposed to work. It's good for the theme developer if we support this – you should check them out here.