Add letter-spacing into sass settings of Foundation framework (6)?

Question

$header-styles: (
  \'small\': (
    \'h1\': (\'fs\': 24, \'lh\': $header-lineheight, \'mt\': 0, \'mb\': $header-margin-bottom),
    \'h2\': (\'fs\': 20, \'lh\': $he