z-index()

@mixin z-index($level: '0', $adjust: 0, $important: '') {
  @if $adjust > 0 {
    z-index: calc(get-var('z-index', $suffix: $level) + $adjust) important($important); // TODO: Replace `get-var()`
  } @else {
    z-index: get-var('z-index', $suffix: $level) important($important);
  }
}