change tab border color

src/client/styles/scss/theme/island.scss

@@ -59,6 +59,9 @@ html[dark] {
   // Sidebar list group
   $bgcolor-sidebar-list-group: #eff8f7; // optional
 
+  // Tabs
+  $bordercolor-nav-tabs: #ccc; // optional
+
   // Icon colors
   $color-editor-icons: $color-global;