message: use Box.defaultProps.borderColor in BorderBox
diff: +import Box from './Box'
  import {withSystemProps, LAYOUT} from './system-props'
  const BorderBox = withSystemProps(
  @@ -5,7 +6,7 @@ const BorderBox = withSystemProps(
  is: 'div',
  bg: 'white',
  border: 1,
  - borderColor: 'gray.2',
  + borderColor: Box.defaultProps.borderColor,
  borderRadius: 1
  },
  LAYOUT
  