diff --git a/css/style.css b/css/style.css
index f0bec25..a89a290 100644
--- a/css/style.css
+++ b/css/style.css
@@ -7,6 +7,26 @@
   --ground: #f5f8fb;
   --panel: #ffffff;
   --line: #dde4ec;
+  --img-bg: #ffffff;
+  --bg-blue-tint: #eef4f9;
+  --bg-green-tint: #f5f9ec;
+  --bg-error: #faf0ed;
+  --border-error: #c0563c;
+}
+[data-theme="dark"] {
+  --blue: #5b9fd4;
+  --blue-dark: #4a87bb;
+  --green: #a8cf35;
+  --ink: #e4edf5;
+  --dim: #8fa0b3;
+  --ground: #0f1923;
+  --panel: #192636;
+  --line: #283a4e;
+  --img-bg: #1e2e3e;
+  --bg-blue-tint: #0a1e2e;
+  --bg-green-tint: #0c1a06;
+  --bg-error: #1e0e0a;
+  --border-error: #d06550;
 }
 * { box-sizing: border-box; }
 body {
@@ -19,8 +39,13 @@ a { color: var(--blue); }
   padding: 14px 24px; background: var(--panel); border-bottom: 1px solid var(--line);
 }
 .brand { display: flex; align-items: center; gap: 10px; font-weight: 700; text-decoration: none; color: var(--blue); font-size: 1.05rem; }
-.site-header nav { display: flex; gap: 18px; }
+.site-header nav { display: flex; gap: 18px; align-items: center; }
 .site-header nav a { color: var(--dim); text-decoration: none; }
+.theme-btn {
+  background: none; border: none; cursor: pointer; color: var(--dim);
+  font-size: 1rem; padding: 0; line-height: 1; visibility: hidden;
+}
+.theme-btn:hover { color: var(--ink); }
 main { max-width: 1080px; margin: 0 auto; padding: 32px 24px 64px; }
 .site-footer { text-align: center; color: var(--dim); font-size: .85rem; padding: 24px; border-top: 1px solid var(--line); }
 .loading { color: var(--dim); }
@@ -40,7 +65,7 @@ main { max-width: 1080px; margin: 0 auto; padding: 32px 24px 64px; }
   padding: 14px; text-decoration: none; color: inherit; transition: border-color .15s, box-shadow .15s;
 }
 .device-card:hover { border-color: var(--blue); box-shadow: 0 4px 14px -8px rgba(65,122,171,.5); }
-.device-card img { width: 100%; aspect-ratio: 1; object-fit: contain; border-radius: 6px; background: #fff; }
+.device-card img { width: 100%; aspect-ratio: 1; object-fit: contain; border-radius: 6px; background: var(--img-bg); }
 .device-card h3 { margin: 10px 0 2px; font-size: 1rem; }
 .device-card p { margin: 0; color: var(--dim); font-size: .82rem; }
 .device-card .go { color: var(--blue); font-weight: 600; font-size: .85rem; margin-top: 8px; display: inline-block; }
@@ -48,7 +73,7 @@ main { max-width: 1080px; margin: 0 auto; padding: 32px 24px 64px; }
 /* Device wizard */
 .device-page .back { display: inline-block; margin-bottom: 16px; text-decoration: none; color: var(--dim); }
 .device-head { display: flex; gap: 20px; align-items: center; margin-bottom: 20px; }
-.device-head img { width: 110px; height: 110px; object-fit: contain; background: #fff; border: 1px solid var(--line); border-radius: 10px; }
+.device-head img { width: 110px; height: 110px; object-fit: contain; background: var(--img-bg); border: 1px solid var(--line); border-radius: 10px; }
 .device-head h1 { margin: 0; font-size: 1.6rem; }
 .device-head p { margin: 2px 0 6px; color: var(--dim); }
 .device-head .links { font-size: .85rem; }
@@ -67,10 +92,10 @@ esp-web-install-button button, .install-btn {
   background: var(--blue); color: #fff; border: 0; border-radius: 8px;
   padding: 10px 22px; font-size: .95rem; font-weight: 700; cursor: pointer;
 }
-.fallback { border-left: 3px solid var(--blue); background: #eef4f9; border-radius: 0 8px 8px 0; padding: 12px 16px; }
+.fallback { border-left: 3px solid var(--blue); background: var(--bg-blue-tint); border-radius: 0 8px 8px 0; padding: 12px 16px; }
 .fallback ul { margin: 8px 0 0; padding-left: 20px; }
-.release-notes { border-left: 3px solid var(--green); background: #f5f9ec; border-radius: 0 8px 8px 0; padding: 12px 16px; margin-top: 14px; font-size: .88rem; }
+.release-notes { border-left: 3px solid var(--green); background: var(--bg-green-tint); border-radius: 0 8px 8px 0; padding: 12px 16px; margin-top: 14px; font-size: .88rem; }
 .release-notes summary { cursor: pointer; font-weight: 600; }
 .release-notes pre { white-space: pre-wrap; font-family: inherit; margin: 8px 0; }
 .done-check { color: var(--green); font-weight: 700; }
-.error-box { border-left: 3px solid #c0563c; background: #faf0ed; border-radius: 0 8px 8px 0; padding: 12px 16px; }
+.error-box { border-left: 3px solid var(--border-error); background: var(--bg-error); border-radius: 0 8px 8px 0; padding: 12px 16px; }
diff --git a/index.html b/index.html
index 7e521c1..40b9d65 100644
--- a/index.html
+++ b/index.html
@@ -6,6 +6,16 @@
   <title>Apollo Installer</title>
   <meta name="description" content="Flash your Apollo Automation device from the browser.">
   <link rel="icon" href="assets/brand/favicon.png">
+  <script>
+    (function () {
+      try {
+        var t = localStorage.getItem('theme');
+        if (t === 'dark' || (!t && matchMedia('(prefers-color-scheme: dark)').matches)) {
+          document.documentElement.setAttribute('data-theme', 'dark');
+        }
+      } catch (e) {}
+    }());
+  </script>
   <link rel="stylesheet" href="css/style.css">
 </head>
 <body>
@@ -18,6 +28,7 @@
       <a href="https://wiki.apolloautomation.com">Wiki</a>
       <a href="https://apolloautomation.com">Shop</a>
       <a href="https://dsc.gg/ApolloAutomation">Discord</a>
+      <button id="theme-toggle" class="theme-btn" title="Toggle theme" aria-label="Toggle theme">🌙</button>
     </nav>
   </header>
   <main id="app"><p class="loading">Loading devices…</p></main>
diff --git a/js/app.js b/js/app.js
index 40a2d13..a6e4c39 100644
--- a/js/app.js
+++ b/js/app.js
@@ -1,6 +1,7 @@
 import { loadRegistry } from './registry.js';
 import { renderHub } from './views/hub.js';
 import { renderDevice } from './views/device.js';
+import { initThemeToggle } from './theme.js';
 
 const app = document.getElementById('app');
 
@@ -29,3 +30,4 @@ async function route() {
 
 window.addEventListener('hashchange', route);
 route();
+initThemeToggle();
diff --git a/js/theme.js b/js/theme.js
new file mode 100644
index 0000000..c56e10a
--- /dev/null
+++ b/js/theme.js
@@ -0,0 +1,63 @@
+// js/theme.js — theme toggle: System / Dark / Light (cycles on click)
+const THEMES = ['system', 'dark', 'light'];
+const ICONS  = { system: '⬤', dark: '🌙', light: '☀' };
+const LABELS = { system: 'System theme', dark: 'Dark theme',  light: 'Light theme' };
+const NEXT_LABEL = {
+  system: 'Switch to dark theme',
+  dark:   'Switch to light theme',
+  light:  'Switch to system theme',
+};
+
+function storedPref() {
+  try { return localStorage.getItem('theme') || 'system'; } catch (e) { return 'system'; }
+}
+
+function savePref(pref) {
+  try {
+    if (pref === 'system') localStorage.removeItem('theme');
+    else localStorage.setItem('theme', pref);
+  } catch (e) {}
+}
+
+function applyPref(pref) {
+  const root = document.documentElement;
+  const isDark =
+    pref === 'dark' ||
+    (pref === 'system' && matchMedia('(prefers-color-scheme: dark)').matches);
+  if (isDark) root.setAttribute('data-theme', 'dark');
+  else root.removeAttribute('data-theme');
+}
+
+export function initThemeToggle() {
+  const btn = document.getElementById('theme-toggle');
+  if (!btn) return;
+
+  const mq = matchMedia('(prefers-color-scheme: dark)');
+
+  function updateBtn(pref) {
+    btn.textContent = ICONS[pref];
+    btn.setAttribute('aria-label', LABELS[pref]);
+    btn.title = NEXT_LABEL[pref];
+  }
+
+  function setPref(pref) {
+    savePref(pref);
+    applyPref(pref);
+    updateBtn(pref);
+  }
+
+  // Keep system mode in sync when OS preference changes
+  mq.addEventListener('change', () => {
+    if (storedPref() === 'system') applyPref('system');
+  });
+
+  btn.addEventListener('click', () => {
+    const current = storedPref();
+    const next = THEMES[(THEMES.indexOf(current) + 1) % THEMES.length];
+    setPref(next);
+  });
+
+  // Initialise button state (makes it visible)
+  updateBtn(storedPref());
+  btn.style.visibility = '';
+}