antarctic theme border-color adjustment.

src/client/styles/scss/theme/antarctic.scss

@@ -98,7 +98,8 @@ html[dark] {
   $color-editor-icons: $color-global;
 
   // Border colors
-  $border-color-theme: $gray-300; // former: `$navbar-border: $gray-300;`
+  $border-color-theme: $gray-400; // former: `$navbar-border: $gray-300;`
+  $border-color-global: $gray-400;
   $bordercolor-inline-code: #ccc8c8; // optional
 
   // Dropdown colors