Dark content area beneath coop header

Wrap content block in bg-dark-800 so all existing dark-themed
templates render correctly without per-file migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

common/artdag_common/templates/_base.html

@@ -81,8 +81,10 @@
     {# App-specific sub-nav (Runs, Recipes, Effects, etc.) #}
     {% block sub_nav %}{% endblock %}
 
-    <main class="max-w-screen-2xl mx-auto px-4 py-4">
+    <main class="bg-dark-800 text-gray-100 min-h-screen">
+      <div class="max-w-screen-2xl mx-auto px-4 py-4">
         {% block content %}{% endblock %}
+      </div>
     </main>
 
     {% block footer %}{% endblock %}