message: Adds zIndex support to Box
diff: @@ -22,6 +22,7 @@ import {
  right,
  bottom,
  left,
  + zIndex,
  } from 'styled-system';
  import { neutralMargins, overflowScrolling } from 'react/styles/mixins';
  @@ -55,6 +56,7 @@ export const mixin = css`
  ${right}
  ${bottom}
  ${left}
  + ${zIndex}
  ${props => props.overflowScrolling && overflowScrolling}
  `;
  