/// Return z-index values for named layers.
/// @author Diego Versiani
/// @see https://www.sitepoint.com/better-solution-managing-z-index-sass/
/// @param {String} $layer - Name of the layer corresponding to a value in the `$z-layers` list variable.
/// @return {String} - Z-index layer value.
@function z( $layer ) {
    @if not map-has-key( $z-layers, $layer ) {
        @warn "No layer found for `#{$layer}` in $z-layers map. Property omitted.";
    }

    @return map-get( $z-layers, $layer );
}