SEO fixes and style fixes.

templates/thank_you.html

@@ -4,7 +4,9 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{{ t.nav_title }} — {{ t.landing_thank_you_title }}</title>
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg">
     <style>
+        :root { color-scheme: light; }
         *, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }
         body {
             font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, sans-serif;