Changed button background fix to match post.hbs

As per JohnONolan’s suggestion, I have edited the nav to match the code
in post.hbs: https://github.com/TryGhost/Casper/blob/master/post.hbs#L9

page.hbs

@@ -3,10 +3,12 @@
 {{! This is a page template. A page outputs content just like any other post, and has all the same
     attributes by default, but you can also customise it to behave differently if you prefer. }}
 
-<nav class="main-nav no-cover clearfix">
+<header class="main-header post-head {{#if image}}" style="background-image: url({{image}}){{else}}no-cover{{/if}}">
+    <nav class="main-nav {{#if image}}overlay{{/if}} clearfix">
         <a class="back-button icon-arrow-left" href="{{@blog.url}}">Home</a>
         <a class="subscribe-button icon-feed" href="{{@blog.url}}/rss/">Subscribe</a>
     </nav>
+</header>
 
 <main class="content" role="main">