add references to user.css in all templates

contrib/templates/default/ukko.mustache

@@ -9,6 +9,7 @@
     <title> ukko / overboard </title>
     <meta charset="utf-8" />
     <link rel="stylesheet" href="{{prefix}}static/site.css"></link>
+    <style src="{{prefix}}static/user.css" type="text/css" />
     <script type="text/javascript" src="{{prefix}}static/nntpchan.js"></script>
     </head>
   <body>