delete an unnecessary nav border line because of duplication

src/client/styles/scss/theme/christmas.scss

@@ -204,7 +204,6 @@ html[dark] {
 
   .grw-navbar {
     background-image: url('/images/themes/christmas/christmas-navbar.jpg');
-    border-bottom: 4px solid $subthemecolor;
   }
 
   .nav-link {