Upgrade Primer CSS to 16 (#18713)

* Upgrade primer to 16

* Upgrade colors in scss

* Get SCSS to build

* Initial dark mode support

* trashcan -> trash

* More specific search browser test

* Fix link color on homepage

* Update overrides.scss

* Upgrade some colors to primer variables

* Hide dark mode support for now

* Apply suggestions from code review

* Update overrides.scss

includes/graphql-arguments.html

@@ -1,5 +1,5 @@
 {% if field.arguments %}
-<div class="border rounded-1 mt-3 mb-3 p-3 bg-gray-light f5">
+<div class="border rounded-1 mt-3 mb-3 p-3 color-bg-secondary f5">
 <p><b>{% data ui.products.graphql.reference.arguments %}</b></p>
 {% for arg in arguments %}
 <div class="border-top mt-2">