Reduce the number of places the default theme is hardcoded from 6 to 1.

templates/head.pug

@@ -1,3 +1,5 @@
+- var DEFAULT_THEME = "/css/themes/slate.css";
+
 mixin head()
   meta(charset="utf-8")
   meta(name="viewport", content="width=device-width, initial-scale=1.0")
@@ -7,7 +9,10 @@ mixin head()
   title= siteTitle
   link(href="/css/sticky-footer-navbar.css", rel="stylesheet")
   link(href="/css/cytube.css", rel="stylesheet")
-  link(id="usertheme", href="/css/themes/slate.css", rel="stylesheet")
+  link(id="usertheme", href=DEFAULT_THEME, rel="stylesheet")
+
+  script(type="text/javascript").
+    var DEFAULT_THEME = '#{DEFAULT_THEME}';
   script(src="/js/theme.js")
   //[if lt IE 9]
     <script src="https://oss.maxcdn.com/libs/html5shiv/3.7.0/html5shiv.js"></script>