Use the same z-index for the stage add button

src/components/stage-selector/stage-selector.css

@@ -1,5 +1,6 @@
 @import "../../css/units.css";
 @import "../../css/colors.css";
+@import "../../css/z-index.css";
 
 $header-height: calc($stage-menu-height - 2px);
 
@@ -79,6 +80,7 @@ $header-height: calc($stage-menu-height - 2px);
 .add-button {
     position: absolute;
     bottom: 0.75rem;
+    z-index: $z-index-add-button
 }
 
 .raised, .raised .header {