feat(header): three-region layout + dark nav colors

styles/navigation/nav-main-default.css

@@ -65,3 +65,13 @@
     }
   }
 }
+
+.nav-main a,
+.nav-aux a {
+	color: var(--color-on-surface, #e3e1e9);
+}
+
+.nav-main a:hover,
+.nav-aux a:hover {
+	color: var(--color-primary, #b6c4ff);
+}