How to use the rebass.theme function in rebass

import * as Rebass from 'rebass'
import { Link } from 'react-router-dom'
import { Consumer as Refunk } from 'refunk'

const components = Object.assign({}, Rebass, {
  Link,
  Refunk,
  h2: Rebass.Heading,
  h3: Rebass.Heading
})

const theme = Rebass.theme

export default {
  ...components,
  theme
}

export const Slider = R.Slider
export const Small = R.Small
export const Sticky = R.Sticky
export const Subhead = R.Subhead
export const Switch = R.Switch
export const Tab = R.Tab
export const Tabs = R.Tabs
export const Text = R.Text
export const Textarea = R.Textarea
export const Toolbar = R.Toolbar
export const Tooltip = R.Tooltip
export const Truncate = R.Truncate
export const colors = R.colors
export const createColors = R.createColors
export const invertTheme = R.invertTheme
export const theme = R.theme