diff --git a/app.py b/app.py
index 0f8a261..a52638f 100644
--- a/app.py
+++ b/app.py
@@ -134,46 +134,69 @@ HTML = """<!doctype html>
 <meta charset="utf-8"/>
 <title>aukpad</title>
 <style>
-  :root { --line-h: 1.4; }
+  :root {
+    --line-h: 1.4;
+    --bg: #fbfbfb;
+    --text: #111;
+    --border: #ddd;
+    --btn-bg: #fff;
+    --gutter-bg: #f8fafc;
+    --gutter-border: #eee;
+    --gutter-color: #9ca3af;
+    --panel-bg: #fff;
+    --overlay-bg: rgba(0,0,0,.55);
+  }
+  [data-theme="dark"] {
+    --bg: #17181E;
+    --text: #F0F0F0;
+    --border: #3a3b45;
+    --btn-bg: #2a2b33;
+    --gutter-bg: #1e1f28;
+    --gutter-border: #2e2f3a;
+    --gutter-color: #6b7280;
+    --panel-bg: #2a2b33;
+    --overlay-bg: rgba(0,0,0,.75);
+  }
   * { box-sizing: border-box; }
-  html, body { height: 100%; margin: 0; padding: 0; background-color: #fbfbfb; }
+  html, body { height: 100%; margin: 0; padding: 0; background-color: var(--bg); color: var(--text); }
   body { font-family: ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, "Noto Sans", "Apple Color Emoji","Segoe UI Emoji";
          max-width: 1000px; margin: 0 auto; padding: 1rem; display: flex; flex-direction: column; height: 100vh; box-sizing: border-box; }
   header { display:flex; justify-content:space-between; align-items:center; margin-bottom: .5rem; flex-shrink: 0; }
-  a,button { padding:.35rem .6rem; text-decoration:none; border:1px solid #ddd; border-radius:4px; background:#fff; cursor:pointer; }
+  a,button { padding:.35rem .6rem; text-decoration:none; border:1px solid var(--border); border-radius:4px; background:var(--btn-bg); color:var(--text); cursor:pointer; }
   #newpad { background:#000; color:#fff; border:1px solid #000; font-weight:bold; }
   #status { font-size:.9rem; opacity:.7; margin-left:.5rem; }
   #status::before { content: "●"; margin-right: .3rem; color: #ef4444; }
   #status.connected::before { color: #22c55e; }
   #peers { font-size:.95rem; font-weight:bold; margin-left:.5rem; margin-right:.3rem; color:#22c55e; display:none; }
-  #wrap { display:grid; grid-template-columns: max-content 1fr; border:1px solid #ddd; border-radius:4px; overflow:hidden;
+  #wrap { display:grid; grid-template-columns: max-content 1fr; border:1px solid var(--border); border-radius:4px; overflow:hidden;
           flex: 1; }
   #gutter, #t { font: 14px/var(--line-h) ui-monospace, SFMono-Regular, Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace; }
-  #gutter { padding:.5rem .75rem; text-align:right; color:#9ca3af; background:#f8fafc; border-right:1px solid #eee;
+  #gutter { padding:.5rem .75rem; text-align:right; color:var(--gutter-color); background:var(--gutter-bg); border-right:1px solid var(--gutter-border);
             user-select:none; min-width: 3ch; white-space: pre; height: 100%; overflow: hidden; }
   #t { padding:.5rem .75rem; width:100%; height: 100%; resize: none; border:0; outline:0;
-       overflow:auto; white-space: pre; }
+       overflow:auto; white-space: pre; background:var(--bg); color:var(--text); }
   #newpad { margin-left:.5rem; }
-  #info { margin-left:.5rem; color: black; font-size: 0.8rem; }
+  #info { margin-left:.5rem; font-size: 0.8rem; }
   pre { margin: 0; }
   /* Password protection */
-  #lock-btn.locked { background:#fef3c7; border-color:#f59e0b; }
-  #pw-panel { display:none; position:absolute; top:2.5rem; right:0; background:#fff; border:1px solid #ddd;
+  #lock-btn.locked { background:#dbeafe; border-color:#3b82f6; }
+  [data-theme="dark"] #lock-btn.locked { background:#1e3a5f; border-color:#3b82f6; }
+  #pw-panel { display:none; position:absolute; top:2.5rem; right:0; background:var(--panel-bg); border:1px solid var(--border);
               border-radius:6px; padding:.75rem; box-shadow:0 4px 12px rgba(0,0,0,.1); z-index:10; min-width:230px; }
   #pw-panel.open { display:block; }
   #pw-panel label { font-size:.8rem; font-weight:bold; display:block; margin-bottom:.4rem; }
-  #pw-panel input { width:100%; padding:.35rem .5rem; border:1px solid #ddd; border-radius:4px;
-                    margin-bottom:.5rem; font-size:.9rem; font-family:inherit; }
+  #pw-panel input { width:100%; padding:.35rem .5rem; border:1px solid var(--border); border-radius:4px;
+                    margin-bottom:.5rem; font-size:.9rem; font-family:inherit; background:var(--btn-bg); color:var(--text); }
   .pw-btns { display:flex; gap:.4rem; }
   .pw-btns button { flex:1; font-size:.8rem; padding:.3rem .4rem; }
   #pw-msg { font-size:.8rem; margin-top:.4rem; color:#6b7280; min-height:1.2em; }
-  #pw-overlay { display:none; position:fixed; inset:0; background:rgba(0,0,0,.55); z-index:100;
+  #pw-overlay { display:none; position:fixed; inset:0; background:var(--overlay-bg); z-index:100;
                 align-items:center; justify-content:center; }
   #pw-overlay.open { display:flex; }
-  #pw-box { background:#fff; border-radius:8px; padding:1.5rem; width:300px; }
+  #pw-box { background:var(--panel-bg); border-radius:8px; padding:1.5rem; width:300px; }
   #pw-box h2 { margin:0 0 .75rem; font-size:1rem; }
-  #auth-input { width:100%; padding:.45rem .6rem; border:1px solid #ddd; border-radius:4px;
-                font-size:1rem; margin-bottom:.5rem; font-family:inherit; display:block; }
+  #auth-input { width:100%; padding:.45rem .6rem; border:1px solid var(--border); border-radius:4px;
+                font-size:1rem; margin-bottom:.5rem; font-family:inherit; display:block; background:var(--btn-bg); color:var(--text); }
   #auth-error { color:#ef4444; font-size:.85rem; margin-bottom:.5rem; display:none; }
   #auth-submit { width:100%; padding:.45rem; background:#000; color:#fff; border:none;
                  border-radius:4px; font-size:.95rem; cursor:pointer; }
@@ -187,6 +210,7 @@ HTML = """<!doctype html>
     <button id="lock-btn" onclick="togglePwPanel()" title="No password – click to set one">Lock</button>
     <a id="newpad" href="/" target="_blank">New pad</a>
     <a id="info" href="/system/info">Info</a>
+    <button id="theme-btn" onclick="toggleTheme()" title="Toggle dark/light mode">Dark</button>
     <div id="pw-panel">
       <label>Password protection</label>
       <input id="pw-input" type="password" placeholder="New password…" onkeydown="if(event.key==='Enter')setPassword()"/>
@@ -219,6 +243,28 @@ const $ = s => document.querySelector(s);
 const proto = location.protocol === "https:" ? "wss" : "ws";
 const rand = () => Math.random().toString(36).slice(2, 6); // 4 chars
 
+// Theme
+function applyTheme(dark) {
+  document.documentElement.setAttribute('data-theme', dark ? 'dark' : 'light');
+  $('#theme-btn').textContent = dark ? 'Light' : 'Dark';
+}
+function toggleTheme() {
+  const isDark = document.documentElement.getAttribute('data-theme') === 'dark';
+  localStorage.setItem('theme', isDark ? 'light' : 'dark');
+  applyTheme(!isDark);
+}
+(function() {
+  const saved = localStorage.getItem('theme');
+  if (saved) { applyTheme(saved === 'dark'); }
+  else {
+    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+    applyTheme(prefersDark);
+  }
+  window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => {
+    if (!localStorage.getItem('theme')) applyTheme(e.matches);
+  });
+})();
+
 // Derive docId from path; redirect root to random
 let docId = decodeURIComponent(location.pathname.replace(/(^\\/|\\/$)/g, ""));
 if (!docId) { location.replace("/" + rand() + "/"); }
@@ -434,16 +480,39 @@ def get_system_info():
 <meta charset="utf-8"/>
 <title>aukpad - System Info</title>
 <style>
+  :root {{
+    --bg: #fbfbfb;
+    --text: #111;
+    --heading: #333;
+    --section-bg: #f8fafc;
+    --value-bg: #e5e7eb;
+  }}
+  [data-theme="dark"] {{
+    --bg: #17181E;
+    --text: #F0F0F0;
+    --heading: #e0e0e0;
+    --section-bg: #1e1f28;
+    --value-bg: #2a2b33;
+  }}
   body {{ font-family: ui-sans-serif, system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial;
-         max-width: 800px; margin: 2rem auto; padding: 1rem; line-height: 1.6; }}
-  h1, h2 {{ color: #333; }}
-  .info-section {{ background: #f8fafc; padding: 1rem; border-radius: 8px; margin: 1rem 0; }}
+         max-width: 800px; margin: 2rem auto; padding: 1rem; line-height: 1.6;
+         background-color: var(--bg); color: var(--text); }}
+  h1, h2 {{ color: var(--heading); }}
+  .info-section {{ background: var(--section-bg); padding: 1rem; border-radius: 8px; margin: 1rem 0; }}
   .config-item {{ margin: 0.5rem 0; }}
-  .value {{ font-family: monospace; background: #e5e7eb; padding: 0.2rem 0.4rem; border-radius: 4px; }}
+  .value {{ font-family: monospace; background: var(--value-bg); padding: 0.2rem 0.4rem; border-radius: 4px; }}
   .back-link {{ display: inline-block; margin-top: 1rem; padding: 0.5rem 1rem;
                 background: #000; color: #fff; text-decoration: none; border-radius: 8px; }}
   .back-link:hover {{ background: #333; }}
 </style>
+<script>
+  (function() {{
+    const saved = localStorage.getItem('theme');
+    const prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
+    const dark = saved ? saved === 'dark' : prefersDark;
+    document.documentElement.setAttribute('data-theme', dark ? 'dark' : 'light');
+  }})();
+</script>
 </head>
 <body>
   <h1>System Information</h1>
