idiomatic way to share styles in styled-components?

In addition to the posted answer, you can also create a function that accepts props / theme and returns the css``.

styled-components will check the type of the value provided eg: ${shared} and if its a function it will invoke it with the relevant props / theme.

import styled, {css} from 'styled-components';

const shared = ({theme, myProp}) => css`
  color: ${theme.color};
`

/* ------------   */

const Component1 = styled.div`
  ${shared};
  /* more styles ... */
`
const Component2 = styled.div`
  ${shared};
  /* more styles ... */
`

You can either share actual CSS strings or share styled-components components:

• Share CSS strings:

import {css} from 'styled-components'
const sharedStyle = css`
  color: green
`

// then later

const ComponentOne = styled.div`
  ${sharedStyle}
  /* some non-shared styles */
`
const ComponentTwo = styled.div`
  ${sharedStyle}
  /* some non-shared styles */
`

• Share actual styled-components:

const Shared = styled.div`
  color: green;
`

// ... then later

const ComponentOne = styled(Shared)`
  /* some non-shared styles */
`
const ComponentTwo = styled(Shared)`
  /* some non-shared styles */
`

Update: Based on questions in the comments, I created an example to show that passing props to styled-components's css function works the same way as passing props to the components themselves: https://codesandbox.io/s/2488xq91qj?fontsize=14. The official recommendation from styled-components is to always wrap strings you will pass to styled-components in the css tag function. In this example, the Test component receives it background and foreground colors through passed-in props embedded in the cssString variable created by invoking the css function.