I've only used styled-components to any extent for css-in-js, I don't remember it having anything to do with a design system? Can you name some examples, you go

I've only used styled-components to any extent for css-in-js, I don't remember it having anything to do with a design system? Can you name some examples, you got me curious. Do these theme.js files have full sizings, spacings, etc defined?