Setup theme-color in head tags

This is picked up by browsers (mostly mobile browsers)
and used e.g. to style the browser header.
This is of great use especially for dark themes.

closes #67

_layouts/default.html

@@ -5,6 +5,10 @@
     <meta http-equiv="X-UA-Compatible" content="IE=edge">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <link rel="stylesheet" href="{{ '/assets/css/style.css?v=' | append: site.github.build_revision | relative_url }}">
+    <!-- Set theme-color. -->
+    <meta name="theme-color" content="#151515" />
+    <meta name="msapplication-navbutton-color" content="#151515" />
+    <meta name="apple-mobile-web-app-status-bar-style" content="#151515" />
 
 {% seo %}
   </head>