It's Just JavaScript™️ ! (I should really make this our official tagline…)

Define your variables like any other module

// constant.js

const constants = {
    primary: '#000',
    lang: 'en'
}

export default constants

and then in your components just use that module

// Button.js
import constants from '../constants'
import styled from 'styled-components'

export default styled.button`
    color: ${constants.primary};
`

No magic to be seen here!