locomotive-boilerplate/assets/styles/settings/_config.zindexes.scss

// ==========================================================================
// Settings / Config / Z-indexes
// ==========================================================================

// Timings
// ==========================================================================

$z-indexes: (
    "header":   200,
    "above":    1,
    "below":    -1
);

// Default z-index for z()
$z-index-default: "above" !default;

// Function
// ==========================================================================

// Retrieves the z-index from the {@see $layers master list}.
//
// @link on http://css-tricks.com/handling-z-index/
//
// @param  {string} $layer    The name of the z-index.
// @param  {number} $modifier A positive or negative modifier to apply
//     to the returned z-index value.
// @throw  Error if the $layer does not exist.
// @throw  Warning if the $modifier might overlap another master z-index.
// @return {number}           The computed z-index of $layer and $modifier.

@function z($layer: $z-index-default, $modifier: 0) {
    @if not map-has-key($z-indexes, $layer) {
        @error "Unknown master z-index layer: #{$layer}";
    }

    @if ($modifier >= 50 or $modifier <= -50) {
        @warn "Modifier may overlap the another master z-index layer: #{$modifier}";
    }

    $index: map-get($z-indexes, $layer);

    @return $index + $modifier;
}