z-index mixin

z.scss

@function z($name) {
    @if index($z-indexes, $name) {
        @return (length($z-indexes) - index($z-indexes, $name)) + 1;
    } @else {
        @warn 'There is no item "#{$name}" in this list; choose one of: #{$z-indexes}';
        @return null;
    }
}
$z-indexes: (
    "outdated-browser",
    "modal",
    "site-header",
    "page-wrapper",
    "site-footer"
);