Fix hidden icon when nav menu doesn't fit the page

If the navigation menu height is greater than the window height and
the menu is minimized, the locale and theme selection icons are
not displayed. This commit fixes this issue by modifying the z-index.

backend/app/assets/stylesheets/spree/backend/components/_navigation.scss

@@ -333,6 +333,14 @@ nav.menu {
   bottom: 0;
 }
 
+body.admin-nav-hidden .admin-nav:not(.fits) {
+  .admin-locale-selection, .admin-theme-selection {
+    &::after {
+      z-index: 0;
+    }
+  }
+}
+
 .admin-login-nav {
   list-style: none;
   padding: 0;