diff --git a/site_static/dark-mode.css b/site_static/dark-mode.css
new file mode 100644
index 0000000..8db627b
--- /dev/null
+++ b/site_static/dark-mode.css
@@ -0,0 +1,378 @@
+/**
+ * Dark Mode Theme for Crypt Server
+ * 
+ * This stylesheet provides dark mode support using CSS custom properties.
+ * Theme automatically respects system preferences via prefers-color-scheme.
+ * Users can also toggle manually via the theme switcher button.
+ */
+
+/* Light theme (default) */
+:root {
+    --bg-primary: #ffffff;
+    --bg-secondary: #f8f9fa;
+    --bg-tertiary: #e9ecef;
+    --text-primary: #212529;
+    --text-secondary: #6c757d;
+    --text-muted: #868e96;
+    --border-color: #dee2e6;
+    --link-color: #007bff;
+    --link-hover: #0056b3;
+    --navbar-bg: #f8f9fa;
+    --navbar-text: #000000;
+    --card-bg: #ffffff;
+    --table-bg: #ffffff;
+    --table-stripe: #f2f2f2;
+    --table-hover: #e9ecef;
+    --input-bg: #ffffff;
+    --input-border: #ced4da;
+    --code-bg: #444444;
+    --code-text: #ffffff;
+    --btn-primary-bg: #007bff;
+    --btn-primary-text: #ffffff;
+    --dropdown-bg: #ffffff;
+    --dropdown-hover: #f8f9fa;
+    --shadow: rgba(0, 0, 0, 0.1);
+}
+
+/* Dark theme */
+[data-theme="dark"] {
+    --bg-primary: #1a1a2e;
+    --bg-secondary: #16213e;
+    --bg-tertiary: #0f3460;
+    --text-primary: #eaeaea;
+    --text-secondary: #a0a0a0;
+    --text-muted: #6c757d;
+    --border-color: #3a3a5c;
+    --link-color: #64b5f6;
+    --link-hover: #90caf9;
+    --navbar-bg: #16213e;
+    --navbar-text: #eaeaea;
+    --card-bg: #16213e;
+    --table-bg: #1a1a2e;
+    --table-stripe: #16213e;
+    --table-hover: #0f3460;
+    --input-bg: #16213e;
+    --input-border: #3a3a5c;
+    --code-bg: #0f3460;
+    --code-text: #e0e0e0;
+    --btn-primary-bg: #0f3460;
+    --btn-primary-text: #ffffff;
+    --dropdown-bg: #16213e;
+    --dropdown-hover: #0f3460;
+    --shadow: rgba(0, 0, 0, 0.3);
+}
+
+/* Auto dark mode based on system preference */
+@media (prefers-color-scheme: dark) {
+    :root:not([data-theme="light"]) {
+        --bg-primary: #1a1a2e;
+        --bg-secondary: #16213e;
+        --bg-tertiary: #0f3460;
+        --text-primary: #eaeaea;
+        --text-secondary: #a0a0a0;
+        --text-muted: #6c757d;
+        --border-color: #3a3a5c;
+        --link-color: #64b5f6;
+        --link-hover: #90caf9;
+        --navbar-bg: #16213e;
+        --navbar-text: #eaeaea;
+        --card-bg: #16213e;
+        --table-bg: #1a1a2e;
+        --table-stripe: #16213e;
+        --table-hover: #0f3460;
+        --input-bg: #16213e;
+        --input-border: #3a3a5c;
+        --code-bg: #0f3460;
+        --code-text: #e0e0e0;
+        --btn-primary-bg: #0f3460;
+        --btn-primary-text: #ffffff;
+        --dropdown-bg: #16213e;
+        --dropdown-hover: #0f3460;
+        --shadow: rgba(0, 0, 0, 0.3);
+    }
+}
+
+/* Apply theme variables to elements */
+body {
+    background-color: var(--bg-primary);
+    color: var(--text-primary);
+    transition: background-color 0.3s ease, color 0.3s ease;
+}
+
+/* Typography */
+h1, h2, h3, h4, h5, h6, h3 a {
+    color: var(--text-primary);
+}
+
+a {
+    color: var(--link-color);
+}
+
+a:hover {
+    color: var(--link-hover);
+}
+
+/* Navbar */
+.navbar {
+    background-color: var(--navbar-bg) !important;
+    border-bottom: 1px solid var(--border-color);
+}
+
+.navbar-brand,
+.navbar-nav .nav-link {
+    color: var(--navbar-text) !important;
+}
+
+.navbar-light .navbar-toggler-icon {
+    filter: var(--text-primary) == #eaeaea ? invert(1) : none;
+}
+
+[data-theme="dark"] .navbar-toggler-icon {
+    filter: invert(1);
+}
+
+/* Dropdowns */
+.dropdown-menu {
+    background-color: var(--dropdown-bg);
+    border-color: var(--border-color);
+}
+
+.dropdown-item {
+    color: var(--text-primary);
+}
+
+.dropdown-item:hover,
+.dropdown-item:focus {
+    background-color: var(--dropdown-hover);
+    color: var(--text-primary);
+}
+
+/* Cards */
+.card {
+    background-color: var(--card-bg);
+    border-color: var(--border-color);
+}
+
+.card-header {
+    background-color: var(--bg-secondary);
+    border-color: var(--border-color);
+    color: var(--text-primary);
+}
+
+/* Tables */
+.table {
+    color: var(--text-primary);
+    background-color: var(--table-bg);
+}
+
+.table thead th {
+    border-color: var(--border-color);
+    background-color: var(--bg-secondary);
+    color: var(--text-primary);
+}
+
+.table td,
+.table th {
+    border-color: var(--border-color);
+}
+
+.table-striped tbody tr:nth-of-type(odd) {
+    background-color: var(--table-stripe);
+}
+
+.table-hover tbody tr:hover {
+    background-color: var(--table-hover);
+    color: var(--text-primary);
+}
+
+/* DataTables */
+.dataTables_wrapper {
+    color: var(--text-primary);
+}
+
+.dataTables_wrapper .dataTables_length,
+.dataTables_wrapper .dataTables_filter,
+.dataTables_wrapper .dataTables_info,
+.dataTables_wrapper .dataTables_paginate {
+    color: var(--text-primary);
+}
+
+.dataTables_wrapper .dataTables_filter input,
+.dataTables_wrapper .dataTables_length select {
+    background-color: var(--input-bg);
+    color: var(--text-primary);
+    border-color: var(--input-border);
+}
+
+.page-item .page-link {
+    background-color: var(--bg-secondary);
+    border-color: var(--border-color);
+    color: var(--text-primary);
+}
+
+.page-item.active .page-link {
+    background-color: var(--btn-primary-bg);
+    border-color: var(--btn-primary-bg);
+}
+
+.page-item.disabled .page-link {
+    background-color: var(--bg-tertiary);
+    color: var(--text-muted);
+}
+
+/* Forms */
+.form-control {
+    background-color: var(--input-bg);
+    border-color: var(--input-border);
+    color: var(--text-primary);
+}
+
+.form-control:focus {
+    background-color: var(--input-bg);
+    color: var(--text-primary);
+    border-color: var(--link-color);
+}
+
+.form-control::placeholder {
+    color: var(--text-muted);
+}
+
+/* Buttons */
+.btn-primary {
+    background-color: var(--btn-primary-bg);
+    border-color: var(--btn-primary-bg);
+    color: var(--btn-primary-text);
+}
+
+.btn-secondary {
+    background-color: var(--bg-tertiary);
+    border-color: var(--border-color);
+    color: var(--text-primary);
+}
+
+.btn-outline-secondary {
+    border-color: var(--border-color);
+    color: var(--text-primary);
+}
+
+.btn-outline-secondary:hover {
+    background-color: var(--bg-secondary);
+    color: var(--text-primary);
+}
+
+/* Code blocks */
+code {
+    background-color: var(--code-bg);
+    color: var(--code-text);
+}
+
+pre {
+    background-color: var(--code-bg);
+    color: var(--code-text);
+    border-color: var(--border-color);
+}
+
+/* Alerts */
+.alert {
+    border-color: var(--border-color);
+}
+
+/* Badges */
+.badge-secondary {
+    background-color: var(--bg-tertiary);
+    color: var(--text-primary);
+}
+
+/* Modals */
+.modal-content {
+    background-color: var(--bg-primary);
+    border-color: var(--border-color);
+}
+
+.modal-header {
+    border-color: var(--border-color);
+}
+
+.modal-footer {
+    border-color: var(--border-color);
+}
+
+.close {
+    color: var(--text-primary);
+}
+
+/* Theme toggle button */
+.theme-toggle {
+    background: none;
+    border: none;
+    cursor: pointer;
+    padding: 0.5rem;
+    color: var(--navbar-text);
+    font-size: 1.2rem;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+}
+
+.theme-toggle:hover {
+    opacity: 0.8;
+}
+
+.theme-toggle .icon-sun,
+.theme-toggle .icon-moon {
+    display: none;
+}
+
+/* Show sun icon in dark mode, moon in light mode */
+[data-theme="dark"] .theme-toggle .icon-sun {
+    display: inline;
+}
+
+[data-theme="dark"] .theme-toggle .icon-moon {
+    display: none;
+}
+
+:root:not([data-theme="dark"]) .theme-toggle .icon-sun {
+    display: none;
+}
+
+:root:not([data-theme="dark"]) .theme-toggle .icon-moon {
+    display: inline;
+}
+
+/* System preference dark mode icon visibility */
+@media (prefers-color-scheme: dark) {
+    :root:not([data-theme="light"]) .theme-toggle .icon-sun {
+        display: inline;
+    }
+    :root:not([data-theme="light"]) .theme-toggle .icon-moon {
+        display: none;
+    }
+}
+
+/* Auto mode indicator - shows "A" badge when following system */
+.theme-toggle.auto-mode::after {
+    content: 'A';
+    font-size: 0.6rem;
+    position: absolute;
+    top: 2px;
+    right: 2px;
+    background: var(--link-color);
+    color: white;
+    border-radius: 50%;
+    width: 12px;
+    height: 12px;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    font-weight: bold;
+}
+
+.theme-toggle {
+    position: relative;
+}
+
+/* Tooltip for auto mode */
+.theme-toggle[title] {
+    cursor: help;
+}
diff --git a/site_static/theme-toggle.js b/site_static/theme-toggle.js
new file mode 100644
index 0000000..3856248
--- /dev/null
+++ b/site_static/theme-toggle.js
@@ -0,0 +1,115 @@
+/**
+ * Theme Toggle Script for Crypt Server
+ *
+ * Handles dark/light mode switching with:
+ * - Automatic system preference detection (prefers-color-scheme)
+ * - Manual override with toggle button
+ * - localStorage persistence of user preference
+ * - "Auto" mode that follows system preference
+ */
+(function() {
+    'use strict';
+
+    const THEME_KEY = 'crypt-theme';
+
+    /**
+     * Get system preference
+     */
+    function getSystemTheme() {
+        return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
+    }
+
+    /**
+     * Get effective theme (what should be displayed)
+     */
+    function getEffectiveTheme() {
+        const stored = localStorage.getItem(THEME_KEY);
+        if (stored === 'auto' || stored === null) {
+            return getSystemTheme();
+        }
+        return stored;
+    }
+
+    /**
+     * Get stored preference (auto, light, or dark)
+     */
+    function getStoredPreference() {
+        return localStorage.getItem(THEME_KEY) || 'auto';
+    }
+
+    /**
+     * Apply theme to document
+     */
+    function applyTheme(theme) {
+        document.documentElement.setAttribute('data-theme', theme);
+        updateToggleButton();
+    }
+
+    /**
+     * Update toggle button icon and label
+     */
+    function updateToggleButton() {
+        const toggleBtn = document.querySelector('.theme-toggle');
+        if (!toggleBtn) return;
+
+        const preference = getStoredPreference();
+        const effective = getEffectiveTheme();
+
+        // Update aria-label based on current state
+        let label;
+        if (preference === 'auto') {
+            label = 'Theme: Auto (following system). Click to switch to ' + 
+                    (effective === 'dark' ? 'light' : 'dark') + ' mode';
+        } else {
+            label = 'Theme: ' + preference + '. Click to switch to auto mode';
+        }
+        toggleBtn.setAttribute('aria-label', label);
+
+        // Update visual indicator for auto mode
+        toggleBtn.classList.toggle('auto-mode', preference === 'auto');
+    }
+
+    /**
+     * Cycle through themes: auto -> light -> dark -> auto
+     */
+    function toggleTheme() {
+        const current = getStoredPreference();
+        let next;
+
+        if (current === 'auto') {
+            // If auto and showing dark, switch to light; if showing light, switch to dark
+            next = getEffectiveTheme() === 'dark' ? 'light' : 'dark';
+        } else if (current === 'light') {
+            next = 'dark';
+        } else {
+            // dark -> auto
+            next = 'auto';
+        }
+
+        localStorage.setItem(THEME_KEY, next);
+        applyTheme(getEffectiveTheme());
+    }
+
+    // Apply theme immediately to prevent flash
+    applyTheme(getEffectiveTheme());
+
+    // Listen for system preference changes
+    window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', function(e) {
+        // Only auto-switch if user preference is 'auto'
+        if (getStoredPreference() === 'auto') {
+            applyTheme(e.matches ? 'dark' : 'light');
+        }
+    });
+
+    // Initialize toggle button when DOM is ready
+    document.addEventListener('DOMContentLoaded', function() {
+        const toggleBtn = document.querySelector('.theme-toggle');
+        if (toggleBtn) {
+            toggleBtn.addEventListener('click', toggleTheme);
+            updateToggleButton();
+        }
+    });
+
+    // Expose toggle function globally
+    window.toggleTheme = toggleTheme;
+})();
diff --git a/templates/base.html b/templates/base.html
index 55bd4c2..22faa2c 100644
--- a/templates/base.html
+++ b/templates/base.html
@@ -18,6 +18,9 @@
   <link rel="stylesheet"
   href='{{ STATIC_URL }}bootstrap/css/bootstrap.min.css'
   type="text/css" media="all" />
+  <link rel="stylesheet"
+  href='{{ STATIC_URL }}dark-mode.css'
+  type="text/css" media="all" />
 {% bootstrap_javascript jquery='full' %}
     <style type="text/css">
       body {
@@ -32,6 +35,8 @@
       <script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script>
     <![endif]-->
 
+    <!-- Theme toggle script (load early to prevent flash) -->
+    <script src="{{ STATIC_URL }}theme-toggle.js"></script>
     <!-- DataTables JavaScript -->
         <script src="{{ STATIC_URL }}dataTables/js/jquery.dataTables.min.js"></script>
         <script src="{{ STATIC_URL }}dataTables/js/dataTables.bootstrap4.min.js"></script>
@@ -75,6 +80,13 @@
                           </li>
 
                           {% endif %}
+                  <!-- Theme toggle button -->
+                  <li class="nav-item">
+                      <button class="theme-toggle nav-link" type="button" aria-label="Toggle dark mode" onclick="toggleTheme()">
+                          <span class="icon-moon">&#9790;</span>
+                          <span class="icon-sun">&#9788;</span>
+                      </button>
+                  </li>
                 </ul>
               </div>
             </nav>