diff --git a/public/app.js b/public/app.js
index c942107..8a0dd07 100644
--- a/public/app.js
+++ b/public/app.js
@@ -1,3 +1,63 @@
+// ============ THEME ============
+const THEME_STORAGE_KEY = 'agentdash-theme';
+const themeToggleBtn = document.getElementById('theme-toggle');
+const systemThemeMedia = window.matchMedia('(prefers-color-scheme: dark)');
+
+function getSystemTheme() {
+    return systemThemeMedia.matches ? 'dark' : 'light';
+}
+
+function getSavedTheme() {
+    try {
+        const saved = localStorage.getItem(THEME_STORAGE_KEY);
+        return saved === 'light' || saved === 'dark' ? saved : null;
+    } catch {
+        return null;
+    }
+}
+
+function setSavedTheme(theme) {
+    try {
+        localStorage.setItem(THEME_STORAGE_KEY, theme);
+    } catch {
+        // Ignore localStorage errors (privacy mode, quota, etc.).
+    }
+}
+
+function updateThemeToggleLabel() {
+    if (!themeToggleBtn) return;
+    const isDarkTheme = document.documentElement.getAttribute('data-theme') === 'dark';
+    const nextTheme = isDarkTheme ? 'light' : 'dark';
+    themeToggleBtn.textContent = isDarkTheme ? 'Light Mode' : 'Dark Mode';
+    themeToggleBtn.setAttribute('aria-label', `Switch to ${nextTheme} mode`);
+}
+
+function applyTheme(theme, { persist = false } = {}) {
+    document.documentElement.setAttribute('data-theme', theme);
+    if (persist) setSavedTheme(theme);
+    updateThemeToggleLabel();
+    if (usageStats) renderUsageCharts();
+}
+
+function initTheme() {
+    const savedTheme = getSavedTheme();
+    applyTheme(savedTheme || getSystemTheme());
+
+    if (themeToggleBtn) {
+        themeToggleBtn.addEventListener('click', () => {
+            const currentTheme = document.documentElement.getAttribute('data-theme') || getSystemTheme();
+            const nextTheme = currentTheme === 'dark' ? 'light' : 'dark';
+            applyTheme(nextTheme, { persist: true });
+        });
+    }
+
+    systemThemeMedia.addEventListener('change', (event) => {
+        if (!getSavedTheme()) {
+            applyTheme(event.matches ? 'dark' : 'light');
+        }
+    });
+}
+
 // ============ NAVIGATION ============
 const navLinks = document.querySelectorAll('.nav-link');
 const pages = document.querySelectorAll('.page');
@@ -559,6 +619,11 @@ function renderUsageStats() {
 }
 
 function renderUsageCharts() {
+    const rootStyles = getComputedStyle(document.documentElement);
+    const themeForeground = rootStyles.getPropertyValue('--fg').trim() || '#e0e0e0';
+    const themeBorder = rootStyles.getPropertyValue('--border').trim() || '#444';
+    const themePrimary = rootStyles.getPropertyValue('--primary').trim() || '#3498db';
+
     // Provider chart
     const providerCtx = document.getElementById('chart-provider').getContext('2d');
     
@@ -583,7 +648,7 @@ function renderUsageCharts() {
             plugins: {
                 legend: {
                     position: 'bottom',
-                    labels: { color: '#e0e0e0' }
+                    labels: { color: themeForeground }
                 }
             }
         }
@@ -604,7 +669,7 @@ function renderUsageCharts() {
             datasets: [{
                 label: 'Requests',
                 data: agentData,
-                backgroundColor: '#3498db'
+                backgroundColor: themePrimary
             }]
         },
         options: {
@@ -617,12 +682,12 @@ function renderUsageCharts() {
             scales: {
                 y: {
                     beginAtZero: true,
-                    ticks: { color: '#e0e0e0' },
-                    grid: { color: '#444' }
+                    ticks: { color: themeForeground },
+                    grid: { color: themeBorder }
                 },
                 x: {
-                    ticks: { color: '#e0e0e0' },
-                    grid: { color: '#444' }
+                    ticks: { color: themeForeground },
+                    grid: { color: themeBorder }
                 }
             }
         }
@@ -708,6 +773,7 @@ function escapeHtml(text) {
 
 // ============ INITIALIZATION ============
 document.addEventListener('DOMContentLoaded', () => {
+    initTheme();
     populateAgentDropdown();
     loadTasks();
     
diff --git a/public/index.html b/public/index.html
index cce2854..ffd6f15 100644
--- a/public/index.html
+++ b/public/index.html
@@ -19,6 +19,7 @@
                 <a href="#wiki" class="nav-link" data-page="wiki">Wiki</a>
                 <a href="#agents" class="nav-link" data-page="agents">Agents</a>
                 <a href="#usage" class="nav-link" data-page="usage">Usage</a>
+                <button id="theme-toggle" class="btn-secondary" type="button" aria-label="Toggle theme">Dark Mode</button>
             </nav>
         </header>
 
diff --git a/public/styles.css b/public/styles.css
index 6fe3a33..f144389 100644
--- a/public/styles.css
+++ b/public/styles.css
@@ -1,11 +1,22 @@
 :root {
-    --bg: #1a1a1a;
-    --fg: #e0e0e0;
-    --border: #444;
+    --bg: #f5f7fb;
+    --fg: #1f2937;
+    --border: #c8d1df;
     --primary: #3498db;
     --secondary: #2ecc71;
     --danger: #e74c3c;
     --warning: #f39c12;
+    --dark: #1f2937;
+    --light: #f8fafc;
+    --card-bg: #ffffff;
+    --card-fg: #111827;
+    --modal-bg: rgba(0, 0, 0, 0.45);
+}
+
+:root[data-theme='dark'] {
+    --bg: #1a1a1a;
+    --fg: #e0e0e0;
+    --border: #444;
     --dark: #121212;
     --light: #f0f0f0;
     --card-bg: #2a2a2a;
@@ -13,6 +24,19 @@
     --modal-bg: rgba(0, 0, 0, 0.7);
 }
 
+@media (prefers-color-scheme: dark) {
+    :root:not([data-theme='light']) {
+        --bg: #1a1a1a;
+        --fg: #e0e0e0;
+        --border: #444;
+        --dark: #121212;
+        --light: #f0f0f0;
+        --card-bg: #2a2a2a;
+        --card-fg: #e0e0e0;
+        --modal-bg: rgba(0, 0, 0, 0.7);
+    }
+}
+
 * {
     margin: 0;
     padding: 0;
@@ -24,6 +48,11 @@ body {
     background-color: var(--bg);
     color: var(--fg);
     line-height: 1.6;
+    transition: background-color 0.3s ease, color 0.3s ease;
+}
+
+body * {
+    transition: background-color 0.3s ease, color 0.3s ease, border-color 0.3s ease, box-shadow 0.3s ease;
 }
 
 .container {
@@ -48,6 +77,7 @@ header h1 {
 nav {
     display: flex;
     gap: 15px;
+    align-items: center;
 }
 
 .nav-link {
@@ -67,6 +97,10 @@ nav {
     color: white;
 }
 
+#theme-toggle {
+    margin-left: auto;
+}
+
 /* Buttons */
 .btn-primary {
     background-color: var(--primary);