Tabs: use primary color as boxShadow color (#14558)

packages/theme-chalk/src/tabs.scss

@@ -109,7 +109,7 @@
     }
 
     &:focus.is-active.is-focus:not(:active) {
-      box-shadow: 0 0 2px 2px #409eff inset;
+      box-shadow: 0 0 2px 2px $--color-primary inset;
       border-radius: 3px;
     }