Add a theory layout.

layouts/theory/single.html

@@ -0,0 +1,29 @@
+{{ define "main" }}
+<h2>{{ .Title | markdownify }}</h2>
+<div class="post-subscript">
+    <p>Last updated on {{ .Date.Format "January 2, 2006" }}.</p>
+</div>
+
+<div class="post-content">
+    {{ if not (eq .TableOfContents "<nav id=\"TableOfContents\"></nav>") }}
+    <div class="table-of-contents">
+        <div class="wrapper">
+            <em>Table of Contents</em>
+            {{ .TableOfContents }}
+        </div>
+    </div>
+    {{ end }}
+
+    {{ if .Draft }}
+    <div class="draft-warning">
+        <em>Warning!</em> This post is a draft. At best, it may contain grammar mistakes;
+        at worst, it can include significant errors and bugs. Please
+        use your best judgement!
+    </div>
+    {{ end }}
+
+    {{ .Content }}
+</div>
+{{ end }}
+{{ define "after" }}
+{{ end }}