add a basic default dark theme

templates/default_dark/header.html

@@ -0,0 +1,9 @@
+<div class="banner">
+  {% if name -%}
+  <div class="site-title">{{name | default(value="unnamed repository") }}</div>
+  <div class="site-description">{{metadata.description | default(value="") }}</div>
+  {% else -%}
+  <div class="site-title">{{site_name | default(value="Itsy-Gitsy") }}</div>
+  <div class="site-description">{{site_description | default(value="A collection of Git repositories")}}</div>
+  {% endif -%}
+</div>