diff --git a/app/globals.css b/app/globals.css
index 02347c6..3d87df0 100644
--- a/app/globals.css
+++ b/app/globals.css
@@ -2,6 +2,72 @@
 @tailwind components;
 @tailwind utilities;
 
+:root {
+  --ce-page: #f9fafb;
+  --ce-surface: #ffffff;
+  --ce-surface-muted: #f9fafb;
+  --ce-surface-elevated: #ffffff;
+  --ce-text: #111827;
+  --ce-text-muted: #6b7280;
+  --ce-border: #d1d5db;
+  --ce-teal: #0f766e;
+  --ce-teal-surface: #f0fdfa;
+  --ce-focus: #14b8a6;
+  --ce-skeleton: #e5e7eb;
+}
+
+html[data-theme="dark"] {
+  --ce-page: #172128;
+  --ce-surface: #202c34;
+  --ce-surface-muted: #1b262e;
+  --ce-surface-elevated: #293740;
+  --ce-text: #edf2f3;
+  --ce-text-muted: #b4c0c5;
+  --ce-border: #40515a;
+  --ce-teal: #62d3c5;
+  --ce-teal-surface: #203a3c;
+  --ce-focus: #78ded2;
+  --ce-skeleton: #3a4a53;
+}
+
+html[data-theme="dark"] body { background-color: var(--ce-page) !important; color: var(--ce-text) !important; }
+html[data-theme="dark"] .app-chrome { background-color: var(--ce-surface-muted); border-color: var(--ce-border); }
+html[data-theme="dark"] .theme-toggle { background-color: var(--ce-surface-elevated); border-color: var(--ce-border); color: var(--ce-text); }
+html[data-theme="dark"] .theme-toggle:hover { background-color: #33444d; }
+html[data-theme="dark"] .bg-white,
+html[data-theme="dark"] [class*="bg-white"] { background-color: var(--ce-surface) !important; }
+html[data-theme="dark"] [class*="bg-gray-50"],
+html[data-theme="dark"] [class*="bg-gray-100"] { background-color: var(--ce-surface-muted) !important; }
+html[data-theme="dark"] [class*="bg-gradient-to"] { background-image: none !important; background-color: var(--ce-surface) !important; }
+html[data-theme="dark"] [class*="bg-teal-50"] { background-color: var(--ce-teal-surface) !important; }
+html[data-theme="dark"] [class*="bg-amber-50"],
+html[data-theme="dark"] [class*="bg-orange-50"],
+html[data-theme="dark"] [class*="bg-yellow-50"] { background-color: #3a3324 !important; }
+html[data-theme="dark"] [class*="bg-green-50"] { background-color: #20392f !important; }
+html[data-theme="dark"] [class*="bg-blue-50"] { background-color: #243540 !important; }
+html[data-theme="dark"] [class*="border-gray"],
+html[data-theme="dark"] [class*="border-teal"],
+html[data-theme="dark"] [class*="border-amber"],
+html[data-theme="dark"] [class*="border-orange"],
+html[data-theme="dark"] [class*="border-green"],
+html[data-theme="dark"] [class*="border-blue"] { border-color: var(--ce-border) !important; }
+html[data-theme="dark"] :is(.text-gray-900, .text-gray-800, .text-gray-700, .text-gray-600) { color: var(--ce-text) !important; }
+html[data-theme="dark"] :is(.text-gray-500, .text-gray-400) { color: var(--ce-text-muted) !important; }
+html[data-theme="dark"] :is(.text-teal-700, .text-teal-800) { color: var(--ce-teal) !important; }
+html[data-theme="dark"] :is(.text-amber-700, .text-amber-800, .text-orange-700, .text-orange-800, .text-yellow-700, .text-yellow-800) { color: #f2c879 !important; }
+html[data-theme="dark"] :is(.text-green-700, .text-green-800) { color: #8bd8a8 !important; }
+html[data-theme="dark"] input,
+html[data-theme="dark"] textarea,
+html[data-theme="dark"] select { background-color: var(--ce-surface-elevated); color: var(--ce-text); border-color: var(--ce-border); }
+html[data-theme="dark"] input::placeholder,
+html[data-theme="dark"] textarea::placeholder { color: #94a3ab; }
+html[data-theme="dark"] details { background-color: var(--ce-surface-muted) !important; }
+html[data-theme="dark"] button:focus-visible,
+html[data-theme="dark"] a:focus-visible,
+html[data-theme="dark"] input:focus-visible,
+html[data-theme="dark"] textarea:focus-visible,
+html[data-theme="dark"] select:focus-visible { outline: 2px solid var(--ce-focus); outline-offset: 2px; }
+
 @keyframes spin {
   from { transform: rotate(0deg); }
   to { transform: rotate(360deg); }
@@ -41,7 +107,7 @@
 .cu-skeleton-line {
   height: 16px;
   border-radius: 8px;
-  background-color: #e5e7eb;
+  background-color: var(--ce-skeleton);
   position: relative;
   overflow: hidden;
 }
diff --git a/app/layout.jsx b/app/layout.jsx
index b601b1b..7bd221c 100644
--- a/app/layout.jsx
+++ b/app/layout.jsx
@@ -1,4 +1,5 @@
 import "./globals.css";
+import ThemeToggle from "@/components/theme-toggle";
 
 export const metadata = {
   title: "Confidence Engine",
@@ -9,6 +10,17 @@ export default function RootLayout({ children }) {
   return (
     <html lang="en">
       <body className="min-h-screen bg-gray-50 text-gray-900">
+        <script
+          dangerouslySetInnerHTML={{
+            __html: `try { const saved = localStorage.getItem('confidence-engine-theme'); const theme = saved === 'dark' || saved === 'light' ? saved : (matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'); document.documentElement.dataset.theme = theme; document.documentElement.style.colorScheme = theme; } catch (_) {}`,
+          }}
+        />
+        <header className="app-chrome border-b border-gray-200/80">
+          <div className="mx-auto flex max-w-[1600px] items-center justify-between px-6 py-3">
+            <span className="text-sm font-semibold tracking-wide text-teal-700">Confidence Engine</span>
+            <ThemeToggle />
+          </div>
+        </header>
         {children}
       </body>
     </html>
diff --git a/components/theme-toggle.jsx b/components/theme-toggle.jsx
new file mode 100644
index 0000000..7aeef2d
--- /dev/null
+++ b/components/theme-toggle.jsx
@@ -0,0 +1,47 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import {
+  readThemePreference,
+  resolveTheme,
+  saveThemePreference,
+  toggleTheme,
+} from "@/lib/theme-preference.js";
+
+function applyTheme(theme) {
+  document.documentElement.dataset.theme = theme;
+  document.documentElement.style.colorScheme = theme;
+}
+
+export default function ThemeToggle() {
+  const [theme, setTheme] = useState("light");
+
+  useEffect(() => {
+    const nextTheme = resolveTheme({
+      savedTheme: readThemePreference(window.localStorage),
+      systemPrefersDark: window.matchMedia?.("(prefers-color-scheme: dark)").matches,
+    });
+    setTheme(nextTheme);
+    applyTheme(nextTheme);
+  }, []);
+
+  const switchTheme = () => {
+    const nextTheme = toggleTheme(theme);
+    setTheme(nextTheme);
+    saveThemePreference(nextTheme, window.localStorage);
+    applyTheme(nextTheme);
+  };
+
+  const isDark = theme === "dark";
+  return (
+    <button
+      type="button"
+      onClick={switchTheme}
+      aria-label={isDark ? "Switch to light mode" : "Switch to dark mode"}
+      className="theme-toggle rounded-lg border px-3 py-2 text-sm font-medium transition focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-teal-500 focus-visible:ring-offset-2"
+    >
+      <span aria-hidden="true" className="mr-1.5">{isDark ? "☀" : "☾"}</span>
+      {isDark ? "Light mode" : "Dark mode"}
+    </button>
+  );
+}
\ No newline at end of file
diff --git a/lib/theme-preference.js b/lib/theme-preference.js
new file mode 100644
index 0000000..3219168
--- /dev/null
+++ b/lib/theme-preference.js
@@ -0,0 +1,22 @@
+export const THEME_STORAGE_KEY = "confidence-engine-theme";
+
+export function resolveTheme({ savedTheme, systemPrefersDark = false } = {}) {
+  if (savedTheme === "light" || savedTheme === "dark") return savedTheme;
+  return systemPrefersDark ? "dark" : "light";
+}
+
+export function toggleTheme(theme) {
+  return theme === "dark" ? "light" : "dark";
+}
+
+export function readThemePreference(storage) {
+  try {
+    return storage?.getItem(THEME_STORAGE_KEY) ?? null;
+  } catch {
+    return null;
+  }
+}
+
+export function saveThemePreference(theme, storage) {
+  storage?.setItem(THEME_STORAGE_KEY, theme);
+}
\ No newline at end of file
diff --git a/tests/theme-preference.test.js b/tests/theme-preference.test.js
new file mode 100644
index 0000000..ce8fcc0
--- /dev/null
+++ b/tests/theme-preference.test.js
@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+import {
+  readThemePreference,
+  resolveTheme,
+  saveThemePreference,
+  THEME_STORAGE_KEY,
+  toggleTheme,
+} from "@/lib/theme-preference.js";
+
+function makeStorage(initial = {}) {
+  const values = new Map(Object.entries(initial));
+  return {
+    getItem: (key) => values.get(key) ?? null,
+    setItem: (key, value) => values.set(key, value),
+  };
+}
+
+describe("theme preference", () => {
+  it("restores a saved dark preference ahead of system preference", () => {
+    const storage = makeStorage({ [THEME_STORAGE_KEY]: "dark" });
+    expect(resolveTheme({ savedTheme: readThemePreference(storage), systemPrefersDark: false })).toBe("dark");
+  });
+
+  it("uses system dark preference when no explicit choice exists", () => {
+    expect(resolveTheme({ savedTheme: null, systemPrefersDark: true })).toBe("dark");
+  });
+
+  it("switches themes and persists the explicit choice", () => {
+    const storage = makeStorage();
+    const nextTheme = toggleTheme("light");
+    saveThemePreference(nextTheme, storage);
+
+    expect(nextTheme).toBe("dark");
+    expect(readThemePreference(storage)).toBe("dark");
+  });
+});
\ No newline at end of file