This has been a labour of love but www.skin-ui.com is now alive and kicking.
If you're using Theme UI for your Gatsby projects Skin UI can help you get started. By forking the default theme you can have a play around with theme styles right in the browser!
There's a preview with some example Theme UI components and the full markdown spec so you can see what affect the theme styles have on the UI.
If you sign up / login you can save your work and come back to it later.
.mdx of the preview.