message: extend Box in BorderBox, remove default bg
diff: import {withSystemProps, LAYOUT} from './system-props'
  +import Box from './Box'
  const BorderBox = withSystemProps(
  {
  - is: 'div',
  - bg: 'white',
  + is: Box,
  border: 1,
  borderColor: 'gray.2',
  borderRadius: 1
  },
  - [...LAYOUT, 'boxShadow']
  + ['boxShadow']
  )
  export default BorderBox
  