Add setting to toggle light/dark mode

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/templates/page/index.html.heex

@@ -7,6 +7,13 @@
     <link rel="apple-touch-icon" href="/img/icons/apple-touch-icon-152x152.png" sizes="152x152" />
     <link rel="mask-icon" href="/img/icons/safari-pinned-tab.svg" color={theme_color()} />
     <meta name="theme-color" content={theme_color()} />
+    <script>
+      if (localStorage.theme === 'dark' || (!('theme' in localStorage) && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
+        document.documentElement.classList.add('dark')
+      } else {
+        document.documentElement.classList.remove('dark')
+      }
+    </script>
     <%= if is_root(assigns) do %>
       <link rel="preload" href="/img/shape-1.svg" as="image" />
       <link rel="preload" href="/img/shape-2.svg" as="image" />