adjust the size of NavbarToggler

src/client/styles/scss/_subnav.scss

@@ -11,6 +11,12 @@
     }
   }
 
+  .grw-navbar-toggler {
+    width: 50px;
+    height: 50px;
+    font-size: 24px;
+  }
+
   h1 {
     @include variable-font-size(32px);
     line-height: 1.4em;