Generated by SassMeister.com.

SassMeister-input-HTML.html

<div id="ORB_BUTTONELEMENT">
  <input id="btnSubmit" type="submit" value="Continue" class="ORB_BUTTON">
</div>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

$zindex-array: (
  'content-overlay',
  'main-nav',
  'header',
  'modal',
  'whatever',
  'footer'
);

@function stack($needle: 'content-overlay', $haystack: $zindex-array) {
	 // create shorthand to reset item.
	 @if $needle == 'default' {
	   @return auto;
	 }
	 // create a shorthand to force item to top of stack.
	 @if $needle == 'force' {
	   @return length($haystack);
	 }
  // check for stack item and return index or error.
	@if index($haystack, $needle) {
	  @return index($haystack, $needle);
	} @else {
	  @warn "`#{inspect($haystack)}` doesn't seem to have a key named `#{inspect($needle)}` setting z-index to 0.";
	  @return 0;
	}
};

.selector {
  z-index: stack('footer');
}

.some-other-selector {
  z-index: stack('sausage');
}

.modal {
  z-index: stack('force');
}

.whatever {
  z-index: stack('default');
}

// If we wanted to do something like create a component...
$modal-stack: ('cloak', 'window', 'title');

.modal {

  z-index: stack('modal');
  
  .cloak {
    z-index: stack('cloak', $modal-stack);
  }
}

SassMeister-output.css

.selector {
  z-index: 6;
}

.some-other-selector {
  z-index: 0;
}

.modal {
  z-index: 6;
}

.whatever {
  z-index: auto;
}

.modal {
  z-index: 4;
}

.modal .cloak {
  z-index: 1;
}

SassMeister-rendered.html

<div id="ORB_BUTTONELEMENT">
  <input id="btnSubmit" type="submit" value="Continue" class="ORB_BUTTON">
</div>