Add dark theme with theme switcher

_layouts/post.html

@@ -4,7 +4,7 @@ layout: default
 <div class="card">
   <div class="row clearfix post-header">
     <div class="column three-fourths">
-      <a href="{{ post.url }}"><h2 class="post-title">{{ page.title }}</h2></a>
+      <h2 class="post-title"><a href="{{ post.url }}">{{ page.title }}</a></h2>
     </div>
     <div class="column fourth">
       <span class="timestamp">{{ page.date | date_to_string }}</span>