$global-elements: (
  content: 100,
  overlay: 400,
);

/// Returns the z-index of the specified element.
/// @param {String} $element - The key for the element.
/// @param {Map} $context - The map in which to search for the element.
/// @return {Number} The z-index for the element.

@function z-index($element, $context: $global-elements) {
  $index: map-get($context, $element);

  @if $index {
    @return $index;
  } @else {
    @error 'z-index `#{$element}` in `#{$context}` not found.';
  }
}