diff --git a/web/index.html b/web/index.html
index 4b126db..331515a 100644
--- a/web/index.html
+++ b/web/index.html
@@ -11,6 +11,35 @@
     --muted: #6b6b6b;
     --rule: #000;
     --soft: #f4f4f4;
+    --active: #222;       /* pressed-button background */
+    --focus-muted: #888;  /* placeholder on inverted input */
+    color-scheme: light;
+  }
+
+  /* Auto dark mode when the user hasn't explicitly chosen light. */
+  @media (prefers-color-scheme: dark) {
+    :root:not([data-theme="light"]) {
+      --fg: #fff;
+      --bg: #000;
+      --muted: #8a8a8a;
+      --rule: #fff;
+      --soft: #0f0f0f;
+      --active: #d8d8d8;
+      --focus-muted: #777;
+      color-scheme: dark;
+    }
+  }
+
+  /* Explicit opt-in via the header toggle wins over OS preference. */
+  :root[data-theme="dark"] {
+    --fg: #fff;
+    --bg: #000;
+    --muted: #8a8a8a;
+    --rule: #fff;
+    --soft: #0f0f0f;
+    --active: #d8d8d8;
+    --focus-muted: #777;
+    color-scheme: dark;
   }
   * { box-sizing: border-box; }
   html, body {
@@ -20,7 +49,7 @@
     font-size: 14px; line-height: 1.55;
     -webkit-font-smoothing: antialiased;
   }
-  ::selection { background: #000; color: #fff; }
+  ::selection { background: var(--fg); color: var(--bg); }
   main {
     max-width: 620px;
     margin: 72px auto 96px;
@@ -76,7 +105,7 @@
     content: "";
     display: inline-block;
     width: 10px; height: 1px;
-    background: #000;
+    background: var(--fg);
   }
 
   label {
@@ -101,10 +130,10 @@
     transition: background 0.08s, color 0.08s;
   }
   input[type=text]:focus {
-    background: #000; color: #fff;
+    background: var(--fg); color: var(--bg);
   }
   input[type=text]::placeholder { color: var(--muted); }
-  input[type=text]:focus::placeholder { color: #888; }
+  input[type=text]:focus::placeholder { color: var(--focus-muted); }
 
   button {
     display: block;
@@ -122,8 +151,8 @@
     border-radius: 0;
     transition: background 0.08s, color 0.08s;
   }
-  button:hover:not(:disabled) { background: #000; color: #fff; }
-  button:active:not(:disabled) { background: #222; }
+  button:hover:not(:disabled) { background: var(--fg); color: var(--bg); }
+  button:active:not(:disabled) { background: var(--active); }
   button:disabled {
     color: var(--muted);
     cursor: not-allowed;
@@ -154,7 +183,7 @@
     white-space: pre-wrap;
   }
   #log .line::before { content: "> "; color: var(--fg); }
-  #log .line.err { color: #000; font-weight: 600; }
+  #log .line.err { color: var(--fg); font-weight: 600; }
   #log .line.err::before { content: "! "; }
 
   footer {
@@ -168,7 +197,26 @@
     justify-content: space-between;
   }
   footer a { color: var(--muted); text-decoration: none; border-bottom: 1px dotted var(--muted); }
-  footer a:hover { color: #000; border-bottom-color: #000; }
+  footer a:hover { color: var(--fg); border-bottom-color: var(--fg); }
+
+  /* Theme toggle — overrides the global button rule via class specificity. */
+  .theme-toggle {
+    margin-left: auto;
+    width: auto;
+    padding: 3px 9px;
+    font-size: 13px;
+    line-height: 1;
+    font-weight: 400;
+    letter-spacing: 0;
+    text-transform: none;
+    border: 1px solid var(--rule);
+    background: var(--bg);
+    color: var(--fg);
+    cursor: pointer;
+    transition: background 0.08s, color 0.08s;
+  }
+  .theme-toggle:hover { background: var(--fg); color: var(--bg); }
+  .theme-toggle:focus-visible { outline: 1px solid var(--fg); outline-offset: 2px; }
 
   .diag {
     border: 1px solid var(--rule);
@@ -187,11 +235,23 @@
   .diag .kv { display: grid; grid-template-columns: 140px 1fr; gap: 4px 12px; }
   .diag .k { color: var(--muted); text-transform: uppercase; letter-spacing: 0.1em; font-size: 10px; }
   .diag .v { word-break: break-all; }
-  .diag .v.ok { color: #000; }
-  .diag .v.bad { color: #000; font-weight: 700; }
+  .diag .v.ok { color: var(--fg); }
+  .diag .v.bad { color: var(--fg); font-weight: 700; }
   .diag .v.bad::before { content: "✗ "; }
   .diag .v.ok::before { content: "✓ "; }
 </style>
+<script>
+  // No-flash theme init: runs synchronously before first paint so a
+  // user who last chose "dark" doesn't see a white flash on load.
+  (function () {
+    try {
+      var t = localStorage.getItem("nostr-passkey:theme");
+      if (t === "dark" || t === "light") {
+        document.documentElement.setAttribute("data-theme", t);
+      }
+    } catch (_) { /* localStorage blocked — fall back to OS preference */ }
+  })();
+</script>
 </head>
 <body>
 <main>
@@ -199,6 +259,7 @@
     <div class="title">
       <h1>nostr-passkey</h1>
       <span class="ver">v0.1 · prototype</span>
+      <button id="btn-theme" class="theme-toggle" type="button" aria-label="Toggle theme" title="Toggle theme">◐</button>
     </div>
     <div class="sub"><b>passkey</b> ⟶ <b>prf</b> ⟶ <b>hkdf</b> ⟶ <b>nostr key</b></div>
   </header>
@@ -531,6 +592,18 @@ async function doDerive() {
 
 document.getElementById("btn-register").addEventListener("click", doRegister);
 document.getElementById("btn-derive").addEventListener("click", doDerive);
+
+// ---------- theme toggle ----------
+function currentTheme() {
+  return document.documentElement.getAttribute("data-theme")
+    || (window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light");
+}
+function toggleTheme() {
+  const next = currentTheme() === "dark" ? "light" : "dark";
+  document.documentElement.setAttribute("data-theme", next);
+  try { localStorage.setItem("nostr-passkey:theme", next); } catch (_) {}
+}
+document.getElementById("btn-theme").addEventListener("click", toggleTheme);
 </script>
 </body>
 </html>