Implement Dark Mode & Custom Themes

Currently, the Tic-Tac-Toe interface has a single light theme, which may not suit all user preferences or lighting conditions. Adding dark mode and customizable color themes will enhance accessibility and personalization. Users should be able to switch between light/dark modes and choose from multiple color palettes such as **pastel**, **neon**, or **classic**.

### 🎯 Proposed Solution
- Introduce a **theme switcher** in the settings menu.
- Provide at least 3–4 predefined color schemes (e.g., pastel, neon, dark classic, retro).
- Allow players to customize **board**, **X/O colors**, and **background**.
- Store user theme preferences locally (using `localStorage` or cookies).

### ✅ Expected Behavior
- Users can toggle between light and dark themes instantly.
- Selected themes persist across sessions.
- All UI elements (board, buttons, background, text) adapt consistently to the chosen theme.
- Smooth transition animation when switching themes for better UX.