Icon Button not centred in navigation bar

I've actually fixed this, thought you'd like to know. Not doing a pull request because the navbar is the only component I'm using.

`  iconContainer: {
    display: 'none',
    [theme.breakpoints.down('sm')]: {
      display: 'block',
      position: 'absolute',
      left: '50%',
      transform: "translate(-50%)"
    }
  },`