smart z-index

z-index.scss

$z-index: (
	navbar: 1,
	content: 2
);

@function z-index($key) {
  @return map-get($z-index, $key);
}

@mixin z-index($key) {
  z-index: z-index($key);
}

.navigation {
  @include z-index(navbar); // 1
}