Here's a few suggestions — please feel free to take them with a grain of salt:
* The "theme" section states: "then use the generated theme in your projects." It might be useful to be more clear that this is formatted for theme-ui on this page by linking to the docs.
* It took a second for the "What body font do you like?" jawn to pop up, so I scrolled down and only scrolled back up because things were moving around on the page.
* I use an admittedly obnoxious color for all of my personal projects (#9604f2) and the utility/extra colors that were provided weren't that great. It might be cool to have each of these colors in the "Here are your colors..." stage have a color picker — as well as when adding your own color.
* I really wish there was a back button instead of just a next button! I picked the wrong body font at first and had to reload the page to go back.
That all being said, I'm going to use the design system that I just generated pretty much immediately!