diff --git a/js/themes.js b/js/themes.js
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+++ b/js/themes.js
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+/**
+ * themes.js - Theme management system
+ */
+
+const STORAGE_KEY_THEME = 'selected-theme';
+
+const themes = {
+  dark: {
+    name: 'Dark (Default)',
+    primary: '#1a1a2e',
+    secondary: '#16213e',
+    accent: '#0f3460',
+    text: '#e4e4e4',
+    textMuted: '#a0a0a0',
+    border: '#2a2a3e',
+    cardBg: 'rgba(30, 30, 46, 0.7)',
+    headerBg: 'rgba(26, 26, 46, 0.85)',
+    overlayBg: 'rgba(60, 60, 60, 0.35)'
+  },
+  light: {
+    name: 'Light',
+    primary: '#f5f5f5',
+    secondary: '#ffffff',
+    accent: '#3498db',
+    text: '#2c3e50',
+    textMuted: '#7f8c8d',
+    border: '#dfe6e9',
+    cardBg: 'rgba(255, 255, 255, 0.85)',
+    headerBg: 'rgba(245, 245, 245, 0.95)',
+    overlayBg: 'rgba(255, 255, 255, 0.15)'
+  },
+  ocean: {
+    name: 'Ocean',
+    primary: '#0a1828',
+    secondary: '#1a2332',
+    accent: '#178582',
+    text: '#e0f4f4',
+    textMuted: '#8ab4b4',
+    border: '#1a3a3a',
+    cardBg: 'rgba(26, 35, 50, 0.75)',
+    headerBg: 'rgba(10, 24, 40, 0.9)',
+    overlayBg: 'rgba(23, 133, 130, 0.15)'
+  },
+  sunset: {
+    name: 'Sunset',
+    primary: '#1a0f1e',
+    secondary: '#2a1b2e',
+    accent: '#d4477a',
+    text: '#fce9e9',
+    textMuted: '#c4a4b4',
+    border: '#3a2a3e',
+    cardBg: 'rgba(42, 27, 46, 0.75)',
+    headerBg: 'rgba(26, 15, 30, 0.9)',
+    overlayBg: 'rgba(212, 71, 122, 0.15)'
+  },
+  forest: {
+    name: 'Forest',
+    primary: '#0f1a0f',
+    secondary: '#1a2a1a',
+    accent: '#4a9a4a',
+    text: '#e4f4e4',
+    textMuted: '#a4c4a4',
+    border: '#2a3a2a',
+    cardBg: 'rgba(26, 42, 26, 0.75)',
+    headerBg: 'rgba(15, 26, 15, 0.9)',
+    overlayBg: 'rgba(74, 154, 74, 0.15)'
+  }
+};
+
+// Apply theme
+function applyTheme(themeName) {
+  const theme = themes[themeName] || themes.dark;
+  const root = document.documentElement;
+  
+  root.style.setProperty('--color-primary', theme.primary);
+  root.style.setProperty('--color-secondary', theme.secondary);
+  root.style.setProperty('--color-accent', theme.accent);
+  root.style.setProperty('--color-text', theme.text);
+  root.style.setProperty('--color-text-muted', theme.textMuted);
+  root.style.setProperty('--color-border', theme.border);
+  root.style.setProperty('--color-card-bg', theme.cardBg);
+  root.style.setProperty('--color-header-bg', theme.headerBg);
+  root.style.setProperty('--color-overlay-bg', theme.overlayBg);
+  
+  // Save preference
+  try {
+    localStorage.setItem(STORAGE_KEY_THEME, themeName);
+  } catch (e) {
+    console.error('Error saving theme:', e);
+  }
+  
+  // Update active state in selector
+  document.querySelectorAll('.theme-option').forEach(option => {
+    option.classList.toggle('active', option.dataset.theme === themeName);
+  });
+}
+
+// Load saved theme
+function loadSavedTheme() {
+  try {
+    const saved = localStorage.getItem(STORAGE_KEY_THEME);
+    return saved || 'dark';
+  } catch (e) {
+    console.error('Error loading theme:', e);
+    return 'dark';
+  }
+}
+
+// Create theme selector UI
+function createThemeSelector() {
+  const selector = document.createElement('div');
+  selector.id = 'theme-selector';
+  selector.className = 'theme-selector';
+  
+  const toggle = document.createElement('button');
+  toggle.className = 'theme-toggle';
+  toggle.innerHTML = '🎨';
+  toggle.title = 'Change Theme';
+  
+  const menu = document.createElement('div');
+  menu.className = 'theme-menu';
+  
+  Object.entries(themes).forEach(([key, theme]) => {
+    const option = document.createElement('div');
+    option.className = 'theme-option';
+    option.dataset.theme = key;
+    option.textContent = theme.name;
+    
+    option.addEventListener('click', () => {
+      applyTheme(key);
+      menu.classList.remove('open');
+    });
+    
+    menu.appendChild(option);
+  });
+  
+  toggle.addEventListener('click', (e) => {
+    e.stopPropagation();
+    menu.classList.toggle('open');
+  });
+  
+  // Close menu when clicking outside
+  document.addEventListener('click', () => {
+    menu.classList.remove('open');
+  });
+  
+  selector.appendChild(toggle);
+  selector.appendChild(menu);
+  
+  return selector;
+}
+
+// Initialize theme system
+function initThemes() {
+  // Apply saved theme
+  const savedTheme = loadSavedTheme();
+  applyTheme(savedTheme);
+  
+  // Add theme selector to header
+  const header = document.querySelector('header');
+  if (header) {
+    const selector = createThemeSelector();
+    header.appendChild(selector);
+  }
+}
+
+// Export for use in other modules
+window.themesModule = {
+  init: initThemes,
+  applyTheme: applyTheme,
+  themes: themes
+};