Add dark mode with system default and toggle

index.html

@@ -3,6 +3,8 @@
 <head>
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
+<meta name="color-scheme" content="light dark">
+<script>try{var t=localStorage.getItem("theme");if(t==="dark"||t==="light")document.documentElement.dataset.theme=t}catch(e){}</script>
 <title>Got Fiber</title>
 <meta name="description" content="Check whether fiber internet is reported at a US address and see broadband contracts and bids within 50 miles.">
 <link rel="icon" href="data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 32 32'%3E%3Ccircle cx='16' cy='16' r='9' fill='%23F0B400' stroke='%2317212B' stroke-width='4'/%3E%3C/svg%3E">
@@ -16,7 +18,10 @@
 <main class="app">
   <aside class="panel">
     <header>
-      <h1>Got Fiber?</h1>
+      <div class="top">
+        <h1>Got Fiber?</h1>
+        <button id="theme" class="theme" type="button" aria-pressed="false">Dark mode</button>
+      </div>
       <p class="lede">Enter a home address to see if fiber internet is reported there, who offers it, and which broadband contracts and bids are within 50 miles.</p>
     </header>
     <form id="search" class="search">