# Dark Mode Implementation

## Overview
The CVS Repository Browser now includes a fully functional dark mode with system theme detection and user preference persistence.

## Features

### 1. **System Theme Detection**
- On first load, the application automatically detects the user's system theme preference using `prefers-color-scheme` media query
- If the system prefers dark mode, the app starts in dark mode
- If the system prefers light mode, the app starts in light mode
- If no system preference is detected, defaults to dark mode

### 2. **User Preference Persistence**
- User's theme choice is saved to browser's localStorage
- On subsequent visits, the saved preference is used instead of system preference
- Users can override system preference by toggling the theme

### 3. **Theme Toggle Button**
- Located in the header next to the status indicator
- Shows 🌙 (moon) icon in light mode
- Shows ☀️ (sun) icon in dark mode
- Click to toggle between light and dark themes

### 4. **Comprehensive Dark Mode Styling**
- All UI elements are properly styled for both light and dark modes
- Uses CSS custom properties (variables) for easy theme switching
- Maintains excellent contrast ratios for accessibility
- Smooth transitions between themes

## Implementation Details

### CSS Variables
The application uses CSS custom properties defined in `:root` and `[data-theme="dark"]` selectors:

**Light Mode (default):**
- Background: `#f9fafb` (light gray)
- Surface: `#ffffff` (white)
- Text Primary: `#111827` (dark gray)
- Text Secondary: `#6b7280` (medium gray)
- Border: `#e5e7eb` (light border)

**Dark Mode:**
- Background: `#1a1a1a` (very dark)
- Surface: `#2d2d2d` (dark gray)
- Text Primary: `#f3f4f6` (light gray)
- Text Secondary: `#d1d5db` (medium light gray)
- Border: `#404040` (dark border)

### JavaScript Implementation
The `UIManager` class in `ui.js` handles theme management:

- `initializeTheme()`: Initializes theme on page load
- `setTheme(theme)`: Sets the theme and updates localStorage
- `toggleTheme()`: Toggles between light and dark modes
- `updateThemeButton(icon)`: Updates the toggle button icon

## Files Modified

1. **styles.css**
   - Added `[data-theme="dark"]` selector with dark mode CSS variables
   - All color references use CSS variables for dynamic theming

2. **index.html**
   - Added theme toggle button in header: `<button id="themeToggleBtn" class="btn-icon">🌙</button>`

3. **ui.js**
   - Added `initializeTheme()` method to detect and apply system theme
   - Added `setTheme()` method to apply theme and save preference
   - Added `toggleTheme()` method to switch between themes
   - Added `updateThemeButton()` method to update button icon
   - Added theme toggle button event listener

## Usage

### For Users
1. The app automatically uses your system theme preference on first visit
2. Click the moon/sun icon in the header to toggle between light and dark modes
3. Your preference is saved and will be remembered on future visits

### For Developers
To modify the dark mode colors, edit the CSS variables in `styles.css`:

```css
[data-theme="dark"] {
    --primary-color: #3b82f6;
    --primary-hover: #2563eb;
    --secondary-color: #9ca3af;
    /* ... other variables ... */
}
```

## Browser Compatibility
- Works in all modern browsers that support:
  - CSS Custom Properties (CSS Variables)
  - `prefers-color-scheme` media query
  - localStorage API
  - ES6 JavaScript

## Testing
A test file `test-dark-mode.html` is included for testing the dark mode functionality independently.

To test:
1. Open `test-dark-mode.html` in a browser
2. Click the moon/sun button to toggle themes
3. Refresh the page to verify persistence
4. Check browser DevTools to see localStorage values