There are a few options here, using composition, styled components, or with props. The second option is probably what you want, but I've provided two other options as well.
1. Using composition
const baseButton = css`
color: white;
background-color: black;
`
const fancyButton = css`
background-color: red;
`
render() {
return (
<div>
<button css={baseButton}></button>
<button css={[baseButton, fancyButton]}></button>
</div>
)
}
The second button will have the baseButton and specialButton styles.
Or...
const baseButton = css`
color: white;
background-color: black;
`
const fancyButton = css`
${baseButton};
background-color: red;
`
render() {
return (
<div>
<button css={baseButton}></button>
<button css={fancyButton}></button>
</div>
)
}
2. Using styled components
const Button = styled.button`
color: white;
background-color: black;
`
const Fancy = styled(Button)`
background-color: red;
`
render() {
return (
<div>
<Button>Button</Button>
<Fancy>Fancy</Fancy>
</div>
)
}
This works for any component that accepts the className prop, which button does.
3. Using props
const Button = styled.button`
color: white;
background-color: ${props => props.fancy ? 'red' : 'black'};
`
render() {
return (
<div>
<Button>Button</Button>
<Button fancy>Fancy</Button>
</div>
)
)