add variables of navbar-default

src/client/styles/scss/_variables-bootstrap.scss

@@ -56,6 +56,11 @@ $navbar-padding-y: 0;
 $navbar-brand-padding-y: 0;
 $navbar-nav-link-padding-x: 1rem;
 
+//== Navbar(variables of bootstrap3)
+$navbar-default-color: #777 !default;
+$navbar-default-bg: #f8f8f8 !default;
+$navbar-default-border: darken($navbar-default-bg, 6.5%) !default;
+
 //== Dropdowns
 $dropdown-border-radius: 0;