Design is an interesting beast.

Good design is not always logical. Color theory, if followed, results in pretty bad experiences. And interestingly, good design can't always be explained in a natural language.

Main thing is, it's very hard to get AI to have taste, because taste is not always statistically explainable.

The best I've gotten to is have it use something like ShadCN (or another well document package that's part of it's training) and make sure that it does two things, only runs the commands to create components, and does not change any stock components or introduce any Tailwind classes for colors and such. Also make it ensure that it maintains the global CSS.

This doesn't make the design look much better than what it is out of the box, but it doesn't turn it into something terrible. If left unprompted on these things, it lands up with mixing fonts that it has absolutely no idea if they look good or not, bringing serif fonts into body text, mixing and matching colors which would have looked really, really good in 2005. But just don't work any more.