add variable name

packages/app/src/styles/_navbar.scss

@@ -1,7 +1,7 @@
 .grw-navbar {
   top: -$grw-navbar-height !important;
 
-  z-index: 1035 !important; // should be higher than SubNavigation
+  z-index: $grw-navbar-z-index-amount !important; // should be higher than SubNavigation
   max-height: $grw-navbar-height + $grw-navbar-border-width;
   border-top: 0;
   border-right: 0;

packages/app/src/styles/_variables.scss

@@ -36,3 +36,7 @@ $grw-logomark-width: 36px;
 $grw-nav-main-left-tab-width: 95px;
 $grw-nav-main-left-tab-width-mobile: 50px;
 $grw-nav-main-tab-height: 42px;
+
+// slightly larger than $zindex-fixed
+// https://getbootstrap.jp/docs/4.5/layout/overview/#z-index
+$grw-navbar-z-index-amount: 1030;