diff --git a/reaction_board/web/static/index.html b/reaction_board/web/static/index.html
index 89b69dd..0518631 100644
--- a/reaction_board/web/static/index.html
+++ b/reaction_board/web/static/index.html
@@ -9,6 +9,7 @@
   <title>咻咻满 Reaction 看板</title>
   <link rel="icon" href="/static/favicon.svg" type="image/svg+xml">
   <link rel="stylesheet" href="/static/style.css">
+  <script src="/static/theme.js"></script>
 </head>
 <body>
   <div class="app" id="app">
@@ -23,6 +24,7 @@
       <div class="topbar-actions">
         <a class="topbar-hint topbar-auth" href="/login" hidden>登录</a>
         <span class="topbar-links" data-topbar-links hidden></span>
+        <button class="theme-toggle" type="button" data-theme-toggle aria-label="切换主题"></button>
       </div>
     </header>
     <div class="preview-banner" id="preview-banner" hidden>预览数据 · 不影响真实数据库</div>
@@ -41,6 +43,7 @@
         <div class="topbar-actions">
           <a class="topbar-hint topbar-auth" href="/login" hidden>登录</a>
           <span class="topbar-links" data-topbar-links hidden></span>
+          <button class="theme-toggle" type="button" data-theme-toggle aria-label="切换主题"></button>
         </div>
       </div>
       <div class="nav-items" id="nav-items"></div>
diff --git a/reaction_board/web/static/style.css b/reaction_board/web/static/style.css
index d874cce..a66736c 100644
--- a/reaction_board/web/static/style.css
+++ b/reaction_board/web/static/style.css
@@ -1,5 +1,5 @@
 :root {
-  color-scheme: light dark;
+  color-scheme: light;
   --bg: #f7f2ea;
   --bg-deep: #efe7db;
   --surface: #fffdf9;
@@ -29,32 +29,32 @@
   --nav-height: 64px;
 }
 
-@media (prefers-color-scheme: dark) {
-  :root {
-    --bg: #1b1411;
-    --bg-deep: #140f0c;
-    --surface: #221a15;
-    --surface-2: #2a201a;
-    --surface-3: #342820;
-    --text: #f3e9dc;
-    --muted: #c9b6a0;
-    --faint: #9c8976;
-    --border: #3a2c22;
-    --grid: #3a2c22;
-    --accent: #e9b97c;
-    --accent-strong: #f6d9ae;
-    --accent-soft: #e9b97c29;
-    --accent-contrast: #2a1a10;
-    --blue: #a8bccc;
-    --blue-soft: #a8bccc1f;
-    --pink: #e3958c;
-    --warning: #e9b97c;
-    --warning-soft: #e9b97c1f;
-    --danger: #e3958c;
-    --success: #a7b98f;
-    --shadow: 0 1px 2px rgba(0, 0, 0, 0.4);
-    --shadow-float: 0 16px 40px rgba(0, 0, 0, 0.55);
-  }
+/* 深色配色由 data-theme 决定；该属性在首屏绘制前由 /static/theme.js 写入。 */
+:root[data-theme="dark"] {
+  color-scheme: dark;
+  --bg: #1b1411;
+  --bg-deep: #140f0c;
+  --surface: #221a15;
+  --surface-2: #2a201a;
+  --surface-3: #342820;
+  --text: #f3e9dc;
+  --muted: #c9b6a0;
+  --faint: #9c8976;
+  --border: #3a2c22;
+  --grid: #3a2c22;
+  --accent: #e9b97c;
+  --accent-strong: #f6d9ae;
+  --accent-soft: #e9b97c29;
+  --accent-contrast: #2a1a10;
+  --blue: #a8bccc;
+  --blue-soft: #a8bccc1f;
+  --pink: #e3958c;
+  --warning: #e9b97c;
+  --warning-soft: #e9b97c1f;
+  --danger: #e3958c;
+  --success: #a7b98f;
+  --shadow: 0 1px 2px rgba(0, 0, 0, 0.4);
+  --shadow-float: 0 16px 40px rgba(0, 0, 0, 0.55);
 }
 
 * { box-sizing: border-box; }
@@ -127,6 +127,25 @@ h1, h2, h3, strong, b { font-weight: 600; }
   font-size: 11px;
 }
 .topbar-actions { display: flex; align-items: center; gap: 8px; flex: 0 0 auto; }
+/* 主题切换：图标表示当前主题，提示文案说明将要切换到的主题。 */
+.theme-toggle {
+  flex: 0 0 auto;
+  display: grid;
+  place-items: center;
+  width: 28px;
+  height: 28px;
+  padding: 0;
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  background: var(--surface);
+  color: var(--muted);
+  cursor: pointer;
+}
+.theme-toggle:hover {
+  color: var(--accent);
+  border-color: color-mix(in srgb, var(--accent) 45%, var(--border));
+}
+.theme-toggle svg { width: 16px; height: 16px; }
 .topbar-links { display: flex; align-items: center; gap: 6px; }
 .topbar-links a {
   padding: 4px 10px;
@@ -167,9 +186,6 @@ h1, h2, h3, strong, b { font-weight: 600; }
   color: var(--accent-contrast);
   background: var(--accent);
 }
-@media (prefers-color-scheme: dark) {
-  .preview-banner { color: var(--accent-contrast); background: var(--accent); }
-}
 
 .main-shell {
   padding: 12px 12px calc(var(--nav-height) + 28px + env(safe-area-inset-bottom));
diff --git a/reaction_board/web/static/theme.js b/reaction_board/web/static/theme.js
new file mode 100644
index 0000000..7f4adb4
--- /dev/null
+++ b/reaction_board/web/static/theme.js
@@ -0,0 +1,90 @@
+// 主题偏好：默认跟随系统；手动切换后写入 localStorage 并固定。
+// 在 <head> 中同步执行，因此在首屏绘制前就写好了 data-theme，不会闪白或闪黑。
+// 服务端 CSP 为 script-src 'self'，不允许内联脚本，所以单独成文件。
+(function () {
+  const KEY = "reaction-board:theme";
+  const root = document.documentElement;
+  const system = window.matchMedia("(prefers-color-scheme: dark)");
+  const ICONS = {
+    light:
+      '<circle cx="12" cy="12" r="4"/><path d="M12 2v2M12 20v2M4.9 4.9l1.4 1.4' +
+      'M17.7 17.7l1.4 1.4M2 12h2M20 12h2M4.9 19.1l1.4-1.4M17.7 6.3l1.4-1.4"/>',
+    dark: '<path d="M21 12.8A9 9 0 1 1 11.2 3a7 7 0 0 0 9.8 9.8z"/>',
+  };
+
+  function stored() {
+    try {
+      const value = window.localStorage.getItem(KEY);
+      return value === "dark" || value === "light" ? value : null;
+    } catch (error) {
+      return null; // 隐私模式等禁用 localStorage 时按跟随系统处理
+    }
+  }
+
+  function resolve() {
+    return stored() || (system.matches ? "dark" : "light");
+  }
+
+  function syncMeta(theme) {
+    // 无 media 的 theme-color 位于最前，优先于 index.html 里按系统匹配的两条，
+    // 这样手动切换后浏览器界面颜色也会跟着变。
+    const fallback = theme === "dark" ? "#1b1411" : "#f7f2ea";
+    const value = getComputedStyle(root).getPropertyValue("--bg").trim() || fallback;
+    let meta = document.querySelector('meta[name="theme-color"]:not([media])');
+    if (!meta) {
+      meta = document.createElement("meta");
+      meta.name = "theme-color";
+      document.head.prepend(meta);
+    }
+    meta.content = value;
+  }
+
+  function syncButtons(theme) {
+    const target = theme === "dark" ? "浅色" : "深色";
+    document.querySelectorAll("[data-theme-toggle]").forEach((button) => {
+      button.innerHTML =
+        '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" ' +
+        `stroke-linecap="round" stroke-linejoin="round" aria-hidden="true">${ICONS[theme]}</svg>`;
+      button.setAttribute("aria-label", `切换到${target}主题`);
+      button.title = `切换到${target}主题`;
+    });
+  }
+
+  function apply() {
+    const theme = resolve();
+    root.dataset.theme = theme;
+    syncMeta(theme);
+    syncButtons(theme);
+  }
+
+  function toggle() {
+    const next = resolve() === "dark" ? "light" : "dark";
+    try {
+      window.localStorage.setItem(KEY, next);
+    } catch (error) {
+      // 无法持久化时仍切换当前页面，下次进入回到跟随系统
+    }
+    apply();
+  }
+
+  // 此时按钮还没解析出来，先锁定首屏配色。
+  apply();
+
+  function bind() {
+    document.querySelectorAll("[data-theme-toggle]").forEach((button) => {
+      button.addEventListener("click", toggle);
+    });
+    apply();
+  }
+
+  if (document.readyState === "loading") {
+    document.addEventListener("DOMContentLoaded", bind, { once: true });
+  } else {
+    bind();
+  }
+
+  // 未手动选择时跟随系统主题变化实时切换。
+  system.addEventListener("change", () => {
+    if (!stored()) apply();
+  });
+})();
\ No newline at end of file
diff --git a/tests/test_web_server.py b/tests/test_web_server.py
index 5429a52..5d53fd2 100644
--- a/tests/test_web_server.py
+++ b/tests/test_web_server.py
@@ -87,7 +87,7 @@ def test_readonly_routes_and_static():
             assert "text/html" in headers["Content-Type"]
             assert b"<html" in payload.lower()
 
-            for asset in ("/static/style.css", "/static/app.js", "/static/vendor/echarts.min.js"):
+            for asset in ("/static/style.css", "/static/app.js", "/static/theme.js", "/static/vendor/echarts.min.js"):
                 status, asset_headers, asset_payload = running.request(asset)
                 assert status == 200, asset
                 assert asset_payload, asset