crispy-css/scss/functions/_zIndex.scss

/**
 * z-index
 *
 * handle z-index
 *
 *
 * @author Björn Hase
 *
 */

$z-indexes: (
    'site-header': 100,
    'modal': 1000
) !default;

/**
 * z-index
 *
 *
 *
 * @return
 *
 */
@function zIndex($name) {
    @if index($z-indexes, $name) {
        @return map($z-indexes, $name);
    } @else {
        @warn 'There is no item "#{$name}" in this list; choose one of: #{$z-indexes}';
        @return null;
    }
}