diff --git a/README.md b/README.md
index e81d2f6..a3a494e 100644
--- a/README.md
+++ b/README.md
@@ -219,7 +219,7 @@ To make DocForge truly user-friendly, include these instructions in the starter
 ## Extensibility Roadmap
 
 - **Plugins**: JS files in `plugins/` with hooks (e.g., add shortcodes, post-build tasks).
-- **Themes**: Override CSS in `assets/custom.css`.
+- **Themes**: Customize variables in `assets/theme.css` or override styles in `assets/custom.css`. A built-in dark-mode toggle stores preference in local storage.
 - **Future**: PDF export plugin, AI-assisted search suggestions.
 - **Community**: MIT license; GitHub for issues.
 
diff --git a/assets/theme.css b/assets/theme.css
new file mode 100644
index 0000000..b6ede2a
--- /dev/null
+++ b/assets/theme.css
@@ -0,0 +1,51 @@
+:root {
+  --bg-color: #ffffff;
+  --text-color: #333333;
+  --sidebar-bg: #f3f3f3;
+  --sidebar-width: 240px;
+}
+[data-theme="dark"] {
+  --bg-color: #222222;
+  --text-color: #eeeeee;
+  --sidebar-bg: #333333;
+}
+body {
+  margin: 0;
+  background: var(--bg-color);
+  color: var(--text-color);
+  font-family: Arial, sans-serif;
+}
+.header {
+  display: flex;
+  align-items: center;
+  padding: 0.5rem 1rem;
+  background: var(--sidebar-bg);
+}
+.logo { text-decoration: none; color: var(--text-color); font-weight: bold; }
+.sidebar-toggle,
+.theme-toggle { background: none; border: none; font-size: 1.2rem; margin-right: 1rem; cursor: pointer; }
+.container { display: flex; }
+.sidebar {
+  width: var(--sidebar-width);
+  background: var(--sidebar-bg);
+  padding: 1rem;
+}
+.sidebar ul { list-style: none; padding: 0; }
+.sidebar a { text-decoration: none; color: var(--text-color); }
+main {
+  flex: 1;
+  padding: 1rem;
+}
+.breadcrumbs a { color: var(--text-color); text-decoration: none; }
+@media (max-width: 768px) {
+  .sidebar {
+    position: fixed;
+    left: -100%;
+    top: 0;
+    height: 100%;
+    overflow-y: auto;
+    transition: left 0.3s ease;
+    z-index: 1000;
+  }
+  body.sidebar-open .sidebar { left: 0; }
+}
diff --git a/assets/theme.js b/assets/theme.js
new file mode 100644
index 0000000..162bc88
--- /dev/null
+++ b/assets/theme.js
@@ -0,0 +1,33 @@
+document.addEventListener('DOMContentLoaded', () => {
+  const sidebarToggle = document.getElementById('sidebar-toggle');
+  const themeToggle = document.getElementById('theme-toggle');
+  const root = document.documentElement;
+
+  function setTheme(theme) {
+    root.dataset.theme = theme;
+    localStorage.setItem('theme', theme);
+  }
+  const stored = localStorage.getItem('theme');
+  if (stored) setTheme(stored);
+
+  sidebarToggle?.addEventListener('click', () => {
+    document.body.classList.toggle('sidebar-open');
+  });
+
+  themeToggle?.addEventListener('click', () => {
+    const next = root.dataset.theme === 'dark' ? 'light' : 'dark';
+    setTheme(next);
+  });
+
+  // breadcrumbs
+  const bc = document.getElementById('breadcrumbs');
+  if (bc) {
+    const parts = location.pathname.split('/').filter(Boolean);
+    let path = '';
+    bc.innerHTML = '<a href="/">Home</a>';
+    parts.forEach((p) => {
+      path += '/' + p;
+      bc.innerHTML += ' / <a href="' + path + '">' + p.replace(/-/g, ' ') + '</a>';
+    });
+  }
+});
diff --git a/src/generator/index.js b/src/generator/index.js
index 3af63e5..4b7554d 100644
--- a/src/generator/index.js
+++ b/src/generator/index.js
@@ -88,7 +88,11 @@ async function generate({ contentDir = 'content', outputDir = '_site', configPat
 
   const elev = new Eleventy(contentDir, outputDir);
   elev.setConfig({
-    dir: { input: contentDir, output: outputDir },
+    dir: {
+      input: contentDir,
+      output: outputDir,
+      includes: path.relative(contentDir, 'templates')
+    },
     templateFormats: ['md', 'njk'],
     markdownTemplateEngine: 'njk',
     htmlTemplateEngine: 'njk',
@@ -97,6 +101,8 @@ async function generate({ contentDir = 'content', outputDir = '_site', configPat
   elev.configFunction = function(eleventyConfig) {
     eleventyConfig.addGlobalData('navigation', nav);
     eleventyConfig.addGlobalData('config', config);
+    eleventyConfig.addGlobalData('layout', 'layout.njk');
+    eleventyConfig.addPassthroughCopy({ 'assets': 'assets' });
   };
   await elev.write();
 
diff --git a/templates/layout.njk b/templates/layout.njk
new file mode 100644
index 0000000..7f96dca
--- /dev/null
+++ b/templates/layout.njk
@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<html lang="en" data-theme="{{ config.theme.darkMode ? 'dark' : 'light' }}">
+<head>
+  <meta charset="UTF-8" />
+  <meta name="viewport" content="width=device-width, initial-scale=1" />
+  <title>{{ title or config.site.title }}</title>
+  <link rel="stylesheet" href="/assets/theme.css" />
+</head>
+<body>
+  {% include "partials/header.njk" %}
+  <div class="container">
+    {% include "partials/sidebar.njk" %}
+    <main id="content">
+      <nav id="breadcrumbs" class="breadcrumbs"></nav>
+      {{ content | safe }}
+    </main>
+  </div>
+  {% include "partials/footer.njk" %}
+  <script src="/assets/theme.js"></script>
+</body>
+</html>
diff --git a/templates/partials/footer.njk b/templates/partials/footer.njk
new file mode 100644
index 0000000..88a57e3
--- /dev/null
+++ b/templates/partials/footer.njk
@@ -0,0 +1,3 @@
+<footer class="footer">
+  <p>&copy; {{ config.site.title }} {{ new Date().getFullYear() }}</p>
+</footer>
diff --git a/templates/partials/header.njk b/templates/partials/header.njk
new file mode 100644
index 0000000..9236c5a
--- /dev/null
+++ b/templates/partials/header.njk
@@ -0,0 +1,5 @@
+<header class="header">
+  <button id="sidebar-toggle" class="sidebar-toggle" aria-label="Toggle navigation">☰</button>
+  <a href="/" class="logo">{{ config.site.title }}</a>
+  <button id="theme-toggle" class="theme-toggle" aria-label="Toggle dark mode">🌓</button>
+</header>
diff --git a/templates/partials/sidebar.njk b/templates/partials/sidebar.njk
new file mode 100644
index 0000000..e22cf10
--- /dev/null
+++ b/templates/partials/sidebar.njk
@@ -0,0 +1,9 @@
+<aside class="sidebar" id="sidebar">
+  <nav>
+    <ul>
+    {% for item in navigation %}
+      <li><a href="{{ item.path }}">{{ item.page.title }}</a></li>
+    {% endfor %}
+    </ul>
+  </nav>
+</aside>