diff --git a/src/index.html b/src/index.html
index 9e84f20..2d3bbf4 100644
--- a/src/index.html
+++ b/src/index.html
@@ -72,6 +72,7 @@
         <div class="settings-tabs">
           <button class="tab-btn active" data-tab="api">API</button>
           <button class="tab-btn" data-tab="character">Character</button>
+          <button class="tab-btn" data-tab="appearance">Appearance</button>
         </div>
 
         <div id="api-tab" class="tab-content active">
@@ -342,6 +343,36 @@
             </div>
           </form>
         </div>
+
+        <div id="appearance-tab" class="tab-content">
+          <div class="settings-form">
+            <div class="form-group">
+              <label for="theme-select">Theme</label>
+              <select id="theme-select" class="theme-select">
+                <option value="dark">Dark (Default)</option>
+                <option value="darker">Darker</option>
+                <option value="midnight">Midnight Blue</option>
+                <option value="forest">Forest</option>
+                <option value="sunset">Sunset</option>
+                <option value="light">Light</option>
+              </select>
+              <small style="color: var(--text-secondary); margin-top: 4px; display: block;">Choose your preferred color scheme</small>
+            </div>
+
+            <div class="theme-preview-container">
+              <div class="theme-preview-label">Preview</div>
+              <div class="theme-preview">
+                <div class="theme-preview-message user-preview">
+                  <div class="theme-preview-content">User message</div>
+                </div>
+                <div class="theme-preview-message assistant-preview">
+                  <div class="theme-preview-avatar"></div>
+                  <div class="theme-preview-content">Assistant response</div>
+                </div>
+              </div>
+            </div>
+          </div>
+        </div>
       </div>
 
       <footer class="input-container">
diff --git a/src/main.js b/src/main.js
index 3721951..3f6fb5f 100644
--- a/src/main.js
+++ b/src/main.js
@@ -14,6 +14,138 @@ let newCharacterBtn;
 let currentCharacter = null;
 let pendingAvatarPath = null;
 
+// Theme definitions
+const themes = {
+  dark: {
+    name: 'Dark (Default)',
+    bgPrimary: '#1a1a1a',
+    bgSecondary: '#252525',
+    bgTertiary: '#2f2f2f',
+    textPrimary: '#e8e8e8',
+    textSecondary: '#a0a0a0',
+    accent: '#6366f1',
+    accentHover: '#4f46e5',
+    userMsg: '#4f46e5',
+    assistantMsg: '#2f2f2f',
+    border: '#3a3a3a',
+    gradient: 'linear-gradient(135deg, #1a1a1a 0%, #2a1a2a 100%)',
+    glow: 'rgba(99, 102, 241, 0.1)'
+  },
+  darker: {
+    name: 'Darker',
+    bgPrimary: '#0a0a0a',
+    bgSecondary: '#141414',
+    bgTertiary: '#1a1a1a',
+    textPrimary: '#e0e0e0',
+    textSecondary: '#909090',
+    accent: '#7c3aed',
+    accentHover: '#6d28d9',
+    userMsg: '#6d28d9',
+    assistantMsg: '#1a1a1a',
+    border: '#2a2a2a',
+    gradient: 'linear-gradient(135deg, #0a0a0a 0%, #1a0a1a 100%)',
+    glow: 'rgba(124, 58, 237, 0.1)'
+  },
+  midnight: {
+    name: 'Midnight Blue',
+    bgPrimary: '#0f1419',
+    bgSecondary: '#1a2332',
+    bgTertiary: '#243447',
+    textPrimary: '#e6f1ff',
+    textSecondary: '#8892a0',
+    accent: '#3b82f6',
+    accentHover: '#2563eb',
+    userMsg: '#1e40af',
+    assistantMsg: '#243447',
+    border: '#2d3e54',
+    gradient: 'linear-gradient(135deg, #0f1419 0%, #1a2845 100%)',
+    glow: 'rgba(59, 130, 246, 0.1)'
+  },
+  forest: {
+    name: 'Forest',
+    bgPrimary: '#0d1b14',
+    bgSecondary: '#162820',
+    bgTertiary: '#1f352b',
+    textPrimary: '#e8f5e9',
+    textSecondary: '#90a89f',
+    accent: '#10b981',
+    accentHover: '#059669',
+    userMsg: '#047857',
+    assistantMsg: '#1f352b',
+    border: '#2d4a3a',
+    gradient: 'linear-gradient(135deg, #0d1b14 0%, #1a2820 100%)',
+    glow: 'rgba(16, 185, 129, 0.1)'
+  },
+  sunset: {
+    name: 'Sunset',
+    bgPrimary: '#1a1214',
+    bgSecondary: '#261a1e',
+    bgTertiary: '#332228',
+    textPrimary: '#fde8e8',
+    textSecondary: '#b89090',
+    accent: '#f97316',
+    accentHover: '#ea580c',
+    userMsg: '#c2410c',
+    assistantMsg: '#332228',
+    border: '#4a3238',
+    gradient: 'linear-gradient(135deg, #1a1214 0%, #2a1a1e 100%)',
+    glow: 'rgba(249, 115, 22, 0.1)'
+  },
+  light: {
+    name: 'Light',
+    bgPrimary: '#ffffff',
+    bgSecondary: '#f5f5f5',
+    bgTertiary: '#e8e8e8',
+    textPrimary: '#1a1a1a',
+    textSecondary: '#666666',
+    accent: '#6366f1',
+    accentHover: '#4f46e5',
+    userMsg: '#6366f1',
+    assistantMsg: '#f0f0f0',
+    border: '#d0d0d0',
+    gradient: 'linear-gradient(135deg, #ffffff 0%, #f5f0ff 100%)',
+    glow: 'rgba(99, 102, 241, 0.05)'
+  }
+};
+
+// Apply theme
+function applyTheme(themeName) {
+  const theme = themes[themeName];
+  if (!theme) return;
+
+  const root = document.documentElement;
+  root.style.setProperty('--bg-primary', theme.bgPrimary);
+  root.style.setProperty('--bg-secondary', theme.bgSecondary);
+  root.style.setProperty('--bg-tertiary', theme.bgTertiary);
+  root.style.setProperty('--text-primary', theme.textPrimary);
+  root.style.setProperty('--text-secondary', theme.textSecondary);
+  root.style.setProperty('--accent', theme.accent);
+  root.style.setProperty('--accent-hover', theme.accentHover);
+  root.style.setProperty('--user-msg', theme.userMsg);
+  root.style.setProperty('--assistant-msg', theme.assistantMsg);
+  root.style.setProperty('--border', theme.border);
+
+  // Update gradient and glow
+  const appContainer = document.querySelector('.app-container');
+  if (appContainer) {
+    appContainer.style.background = theme.gradient;
+    const glow = appContainer.querySelector('::before');
+  }
+
+  // Store preference
+  localStorage.setItem('claudia-theme', themeName);
+}
+
+// Load saved theme
+function loadSavedTheme() {
+  const savedTheme = localStorage.getItem('claudia-theme') || 'dark';
+  const themeSelect = document.getElementById('theme-select');
+  if (themeSelect) {
+    themeSelect.value = savedTheme;
+  }
+  applyTheme(savedTheme);
+}
+
 // Helper function to get avatar URL
 async function getAvatarUrl(avatarFilename) {
   if (!avatarFilename) return null;
@@ -1039,6 +1171,14 @@ function setupAppControls() {
       section.classList.toggle('collapsed');
     });
   });
+
+  // Setup theme selector
+  const themeSelect = document.getElementById('theme-select');
+  if (themeSelect) {
+    themeSelect.addEventListener('change', (e) => {
+      applyTheme(e.target.value);
+    });
+  }
 }
 
 // Keyboard shortcuts
@@ -1433,5 +1573,8 @@ window.addEventListener('DOMContentLoaded', () => {
   messageInput.focus();
   setStatus('Ready');
 
+  // Load saved theme before anything else
+  loadSavedTheme();
+
   loadExistingConfig();
 });
diff --git a/src/styles.css b/src/styles.css
index 29fc753..163e370 100644
--- a/src/styles.css
+++ b/src/styles.css
@@ -811,25 +811,117 @@ body {
   animation: spin 0.8s linear infinite;
 }
 
-/* Light mode support */
-@media (prefers-color-scheme: light) {
-  :root {
-    --bg-primary: #ffffff;
-    --bg-secondary: #f5f5f5;
-    --bg-tertiary: #e8e8e8;
-    --text-primary: #1a1a1a;
-    --text-secondary: #666666;
-    --accent: #6366f1;
-    --accent-hover: #4f46e5;
-    --user-msg: #6366f1;
-    --assistant-msg: #f5f5f5;
-    --border: #e0e0e0;
-    --shadow: rgba(0, 0, 0, 0.1);
-  }
+/* Theme System */
+/* Dark theme (default) */
+:root,
+[data-theme="dark"] {
+  --bg-primary: #1a1a1a;
+  --bg-secondary: #252525;
+  --bg-tertiary: #2f2f2f;
+  --text-primary: #e8e8e8;
+  --text-secondary: #a0a0a0;
+  --accent: #6366f1;
+  --accent-hover: #4f46e5;
+  --user-msg: #4f46e5;
+  --assistant-msg: #2f2f2f;
+  --border: #3a3a3a;
+  --shadow: rgba(0, 0, 0, 0.5);
+}
 
-  .message.assistant .message-content {
-    border: 1px solid var(--border);
-  }
+/* Light theme */
+[data-theme="light"] {
+  --bg-primary: #ffffff;
+  --bg-secondary: #f5f5f5;
+  --bg-tertiary: #e8e8e8;
+  --text-primary: #1a1a1a;
+  --text-secondary: #666666;
+  --accent: #6366f1;
+  --accent-hover: #4f46e5;
+  --user-msg: #6366f1;
+  --assistant-msg: #f5f5f5;
+  --border: #e0e0e0;
+  --shadow: rgba(0, 0, 0, 0.1);
+}
+
+[data-theme="light"] .app-container {
+  background: linear-gradient(135deg, #ffffff 0%, #f5f5f5 100%);
+}
+
+[data-theme="light"] .app-container::before {
+  background: radial-gradient(circle at top center, rgba(99, 102, 241, 0.05) 0%, transparent 70%);
+}
+
+[data-theme="light"] .message.assistant .message-content {
+  border: 1px solid var(--border);
+}
+
+/* Abyss theme - Deep purple/blue */
+[data-theme="abyss"] {
+  --bg-primary: #0f0f1e;
+  --bg-secondary: #1a1a2e;
+  --bg-tertiary: #252538;
+  --text-primary: #e0e0ff;
+  --text-secondary: #8888bb;
+  --accent: #9d4edd;
+  --accent-hover: #7b2cbf;
+  --user-msg: #7b2cbf;
+  --assistant-msg: #252538;
+  --border: #2f2f48;
+  --shadow: rgba(0, 0, 0, 0.7);
+}
+
+[data-theme="abyss"] .app-container {
+  background: linear-gradient(135deg, #0f0f1e 0%, #1a1033 100%);
+}
+
+[data-theme="abyss"] .app-container::before {
+  background: radial-gradient(circle at top center, rgba(157, 78, 221, 0.15) 0%, transparent 70%);
+}
+
+/* Nord theme - Cool blues and grays */
+[data-theme="nord"] {
+  --bg-primary: #2e3440;
+  --bg-secondary: #3b4252;
+  --bg-tertiary: #434c5e;
+  --text-primary: #eceff4;
+  --text-secondary: #d8dee9;
+  --accent: #88c0d0;
+  --accent-hover: #81a1c1;
+  --user-msg: #5e81ac;
+  --assistant-msg: #434c5e;
+  --border: #4c566a;
+  --shadow: rgba(0, 0, 0, 0.4);
+}
+
+[data-theme="nord"] .app-container {
+  background: linear-gradient(135deg, #2e3440 0%, #3b4252 100%);
+}
+
+[data-theme="nord"] .app-container::before {
+  background: radial-gradient(circle at top center, rgba(136, 192, 208, 0.1) 0%, transparent 70%);
+}
+
+/* Mocha theme - Warm browns */
+[data-theme="mocha"] {
+  --bg-primary: #1e1e2e;
+  --bg-secondary: #2a2837;
+  --bg-tertiary: #3a3850;
+  --text-primary: #cdd6f4;
+  --text-secondary: #bac2de;
+  --accent: #f5c2e7;
+  --accent-hover: #cba6f7;
+  --user-msg: #cba6f7;
+  --assistant-msg: #3a3850;
+  --border: #45475a;
+  --shadow: rgba(0, 0, 0, 0.5);
+}
+
+[data-theme="mocha"] .app-container {
+  background: linear-gradient(135deg, #1e1e2e 0%, #2a2440 100%);
+}
+
+[data-theme="mocha"] .app-container::before {
+  background: radial-gradient(circle at top center, rgba(245, 194, 231, 0.1) 0%, transparent 70%);
 }
 
 /* Settings Panel - Slide-in Sidebar */
@@ -1160,6 +1252,78 @@ body {
   transform: scale(0.95);
 }
 
+/* Theme Preview */
+.theme-preview-container {
+  margin-top: 20px;
+}
+
+.theme-preview-label {
+  font-size: 13px;
+  font-weight: 500;
+  color: var(--text-secondary);
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  margin-bottom: 12px;
+}
+
+.theme-preview {
+  background: var(--bg-tertiary);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+  padding: 16px;
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+}
+
+.theme-preview-message {
+  display: flex;
+  align-items: flex-start;
+  gap: 10px;
+  animation: none;
+}
+
+.theme-preview-message.user-preview {
+  justify-content: flex-end;
+}
+
+.theme-preview-message.assistant-preview {
+  justify-content: flex-start;
+}
+
+.theme-preview-avatar {
+  width: 28px;
+  height: 28px;
+  border-radius: 50%;
+  background: var(--accent);
+  flex-shrink: 0;
+  opacity: 0.6;
+}
+
+.theme-preview-content {
+  padding: 10px 14px;
+  border-radius: 10px;
+  font-size: 13px;
+  max-width: 70%;
+}
+
+.theme-preview-message.user-preview .theme-preview-content {
+  background: linear-gradient(135deg, var(--accent) 0%, var(--accent-hover) 100%);
+  color: white;
+  border-bottom-right-radius: 4px;
+}
+
+.theme-preview-message.assistant-preview .theme-preview-content {
+  background: var(--assistant-msg);
+  color: var(--text-primary);
+  border-bottom-left-radius: 4px;
+  border: 1px solid var(--border);
+}
+
+.theme-select {
+  cursor: pointer;
+}
+
 /* Responsive */
 @media (max-width: 600px) {
   .messages-list {