Add dark mode to website

- Use https://github.com/bufferhead-code/nightowl to provide hooks for
  prefers-color-scheme (OS level choice) and a manual button to toggle
  dark mode
- Replace github logo with icon from font-awesome so that dark mode just
  works

Author: joshka

_layouts/default.html

@@ -15,6 +15,8 @@
   <link rel="stylesheet" href="{{ "/css/rouge-github.css" | prepend: site.baseurl }}">
   <link rel="stylesheet" href="{{ "/css/main.css" | prepend: site.baseurl }}">
   <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.4.0/css/font-awesome.min.css">
+  <!-- dark mode -->
+  <script type="module" src="https://cdn.jsdelivr.net/npm/@bufferhead/[email protected]/dist/nightowl.js"></script>
 
   <link rel="canonical" href="{{ page.url | replace:'index.html','' | prepend: site.baseurl | prepend: site.url }}">
   <link rel="alternate" type="application/rss+xml" title="{{ site.title }}"

assets/github.svg

@@ -3,7 +3,7 @@
 ---
 
 <section class="greeting">
-    <h1>
+    <h1 class="nightowl-daylight">
         <img id="logo" src="/assets/rust-analyzer.svg" alt="rust analyzer">
     </h1>
 
@@ -13,7 +13,7 @@ <h1>
     </p>
 
     <a class="source" href="https://github.com/rust-analyzer/rust-analyzer">
-        <img src="/assets/github.svg" alt="GitHub"> Source
+        <i class="fa fa-github" aria-hidden="true"></i> Source
     </a>
 </section>