What they did
Mendes, a solo builder without a design background, wanted to know whether design skills can move an AI-built interface away from the familiar default look. He wrote one PROJECT.md for a new app, Coffee Nerds, covering the vision, users, a data model for coffees, methods, recipes and brew logs, and six well-known recipes as seed data. With Claude Opus 5.5 and the same execution prompt each time, he had the agent build four versions in separate branches of a public repo: no skill, no skill but using Claude Design, the Taste Skill, and the UI UX Pro Max skill. Each version got one prompt and one run, with no back and forth.
How it went
The default build and the Claude Design build looked like siblings: off-white background, a red pill button, rounded cards and stat tiles, though Claude Design laid out the page more clearly. Taste Skill produced an editorial, black and white look with large type, sourced its own images, video and animation, and wrapped the app in a landing page while still shipping every feature in the spec. UI UX Pro Max gave a coffee-coloured palette, serif headings and sidebar navigation, and wrote a design system and style guide along the way.
Worth knowing
This was one project and one run per version, so results will vary on a rerun. The spec set the structure of every screen; the skill only changed the look. Mendes notes that in his real projects a branding file the agent reads each time leaves little room for a skill to change anything.