> nah. what about theming? you cannot do that with inline styles.

Nobody is utilizing inline styles in that example. It _looks_ like inline styles but it not. It generates css class name behind the hood which you can override in multiple ways. Starting from the ThemeProvider or just like this:

const Footer = styled.div` ${PrimaryButton} { color: red; } `

I strongly suggest to look at https://styled-system.com/ to better understand the mechanics.