docs(user): add themes and wood paneling guide

- Document all available color themes with descriptions
- Explain wood paneling feature and available textures
- Provide step-by-step instructions for changing themes
- Add tips for theme/texture pairing

docs/user/themes.md

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+# Themes and Wood Paneling
+
+## Color Themes
+
+Bookhoard includes multiple color themes to suit your preferences:
+
+- **Tokyo Night** (default) - Dark blue/purple tones
+- **Dracula** - Dark purple accent colors
+- **Nord** - Arctic, bluish-gray colors
+- **Solarized Dark** - Precision contrast for readability
+- **Monokai** - Dark theme with vibrant accents
+- **One Dark Pro** - Atom's favorite theme
+- **Material Dark** - Material Design dark theme
+- **Catppuccin Mocha** - Soothing pastel dark theme
+- **Catppuccin Macchiato** - Warm mid-tone theme
+- **Catppuccin Frappé** - Frothy cool tones
+- **Catppuccin Latte** - Warm light theme
+
+### Changing Your Theme
+
+1. Click the theme icon (palette) in the header
+2. Select your preferred color theme
+3. Your choice is saved automatically and synced across devices
+
+## Wood Paneling
+
+Wood paneling adds texture to your dashboard bookshelf background, giving it a classic bookshelf feel.
+
+### Available Wood Textures
+
+- **None** (default) - Solid color background
+- **Wood Light** - Light oak/birch texture
+- **Wood Dark** - Dark walnut/mahogany texture
+- **Wood Mahogany** - Reddish-brown mahogany texture
+
+### Applying Wood Paneling
+
+1. Click the theme icon (palette) in the header
+2. Scroll to "Bookshelf Background" section
+3. Select your preferred wood texture
+4. Texture is applied to dashboard bookshelf only
+
+**Note:** Wood paneling is a browser preference and is not synced across devices.
+
+### Tips
+
+- Wood textures work best with darker color themes (Tokyo Night, Nord, etc.)
+- Light wood pairs well with light themes (Catppuccin Latte)
+- Wood paneling only affects the dashboard bookshelf area