message: use theme.fonts and theme.lineHeight in CSS
diff: import React from 'react'
  +import {theme} from '../src'
  -const CSS = ({ css }) =>
  - <style
  - dangerouslySetInnerHTML={{
  - __html: css
  - }}
  - />
  +const CSS = ({css}) => (
  + <style dangerouslySetInnerHTML={{__html: css}} />
  +)
  +
  +const fonts = theme.fonts
  + .map(name => name.indexOf(' ') > -1 ? `"${name}"` : name)
  + .join(', ')
  CSS.defaultProps = {
  css: `
  - * { box-sizing: border-box }
  + * { box-sizing: border-box; }
  body {
  margin: 0;
  - font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif;
  - line-height: 1.5;
  + font-family: ${fonts};
  + line-height: ${theme.lineHeight};
  }
  `
  }
  