This is an interesting yet simple approach. To the OP’s original question, how might you abstract this into a “design system” that can be applied to their other projects?
From there you just select the brand at generation time. I've found you don't need a design.md or a npm package - simple screenshots are plenty good enough. Here's a prompt for "a landing page for a new satellite connectivity" feature I generated in reddit/netflix/slack's brands: https://image.non.io/b5e23f19-5041-4f87-9b97-0af39986d1b0.we...