diff --git a/403.html b/403.html
index 41fab77..5f28ab6 100644
--- a/403.html
+++ b/403.html
@@ -5,9 +5,11 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Forbidden · druid520</title>
 <link rel="stylesheet" href="/style.css">
+<script src="/theme.js"></script>
 </head>
 <body>
 <main class="error">
+  <div class="topbar"><button id="theme-toggle" class="theme-toggle" type="button" hidden>Dark mode</button></div>
   <h2>Error 403</h2>
   <h1>Forbidden</h1>
   <p>You don't have access to this page.</p>
diff --git a/404.html b/404.html
index 177e017..38f7a19 100644
--- a/404.html
+++ b/404.html
@@ -5,9 +5,11 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Not found · druid520</title>
 <link rel="stylesheet" href="/style.css">
+<script src="/theme.js"></script>
 </head>
 <body>
 <main class="error">
+  <div class="topbar"><button id="theme-toggle" class="theme-toggle" type="button" hidden>Dark mode</button></div>
   <h2>Error 404</h2>
   <h1>Not found</h1>
   <p>There's nothing at this address.</p>
diff --git a/index.html b/index.html
index 966043d..203d1c6 100644
--- a/index.html
+++ b/index.html
@@ -6,9 +6,11 @@
 <title>druid520</title>
 <meta name="description" content="Contact details and public keys for druid520.">
 <link rel="stylesheet" href="/style.css">
+<script src="/theme.js"></script>
 </head>
 <body>
 <main>
+  <div class="topbar"><button id="theme-toggle" class="theme-toggle" type="button" hidden>Dark mode</button></div>
   <h1>druid520</h1>
 
   <h2>Contact</h2>
diff --git a/style.css b/style.css
index fe78d48..5dc56bd 100644
--- a/style.css
+++ b/style.css
@@ -8,8 +8,9 @@
   color-scheme: light dark;
 }
 
+/* Dark palette: follows the OS unless the switch pinned a theme (data-theme on <html>). */
 @media (prefers-color-scheme: dark) {
-  :root {
+  :root:not([data-theme="light"]) {
     --bg: #111;
     --fg: #e6e6e6;
     --muted: #999;
@@ -19,6 +20,20 @@
   }
 }
 
+:root[data-theme="dark"] {
+  --bg: #111;
+  --fg: #e6e6e6;
+  --muted: #999;
+  --line: #333;
+  --code-bg: #1c1c1c;
+  --link: #7ab7ff;
+  color-scheme: dark;
+}
+
+:root[data-theme="light"] { color-scheme: light; }
+
+[hidden] { display: none !important; }
+
 * { box-sizing: border-box; }
 
 body {
@@ -47,6 +62,22 @@ h2 {
 a { color: var(--link); text-decoration: none; }
 a:hover { text-decoration: underline; }
 
+/* Theme switch */
+.topbar { display: flex; justify-content: flex-end; margin: -1.5rem 0 1.5rem; }
+.theme-toggle {
+  padding: .25rem .7rem;
+  background: transparent;
+  color: var(--fg);
+  border: 1px solid var(--line);
+  border-radius: 6px;
+  font-family: inherit;
+  font-size: .85rem;
+  line-height: 1.4;
+  cursor: pointer;
+}
+.theme-toggle:hover { border-color: var(--muted); }
+.theme-toggle:focus-visible { outline: 2px solid var(--link); outline-offset: 2px; }
+
 /* Contact list */
 dl {
   display: grid;
diff --git a/theme.js b/theme.js
new file mode 100644
index 0000000..f7d2aea
--- /dev/null
+++ b/theme.js
@@ -0,0 +1,35 @@
+// Light/dark switch. The page follows the OS theme until the button is used;
+// the choice is then remembered in localStorage. Loaded in <head> so a saved
+// theme is applied before the page paints.
+(function () {
+  var root = document.documentElement;
+  var media = window.matchMedia('(prefers-color-scheme: dark)');
+
+  try {
+    var saved = localStorage.getItem('theme');
+    if (saved === 'light' || saved === 'dark') root.dataset.theme = saved;
+  } catch (e) {}
+
+  function current() {
+    return root.dataset.theme || (media.matches ? 'dark' : 'light');
+  }
+
+  function label(btn) {
+    var next = current() === 'dark' ? 'light' : 'dark';
+    btn.textContent = next === 'dark' ? 'Dark mode' : 'Light mode';
+    btn.setAttribute('aria-label', 'Switch to ' + next + ' mode');
+  }
+
+  document.addEventListener('DOMContentLoaded', function () {
+    var btn = document.getElementById('theme-toggle');
+    if (!btn) return;
+    label(btn);
+    btn.hidden = false;
+    btn.addEventListener('click', function () {
+      root.dataset.theme = current() === 'dark' ? 'light' : 'dark';
+      try { localStorage.setItem('theme', root.dataset.theme); } catch (e) {}
+      label(btn);
+    });
+    media.addEventListener('change', function () { label(btn); });
+  });
+})();