I chose Rebass as a component library for my app. Because Rebass is unopinionated about theming, it needs a theming library. Fine, I chose Theme UI to define my theme and then I started to build my app.
I quickly noticed that all the React components (Box, Flex, Card, Link, etc.) are available in both Rebass and Theme UI. When browsing both documentations, the differences are hard to tell.
What am I missing if I remove Rebass completely? What is the purpose of this library?
Rebass and theme-ui are different libraries but following the same styled ecosystem. Both of the library has it's own theming capability. You can use the both at the same time though it is not required.