Improve Pong Game UI/UX: Dark Mode, Player Name Display & Styling

The current Pong Game has a basic UI without visual enhancements or personalization. This issue aims to improve the overall user experience with the following enhancements:

🌙 Add Dark Mode: Implement a toggle switch to switch between light and dark themes for better accessibility and user preference.

🧑 Display Player Name: Use the name entered on the welcome screen and dynamically show it in the scoreboard during gameplay.

🎨 Styling Improvements:

Add hover and focus animations to buttons and inputs.

Improve layout spacing and alignment.

Add a clean, modern look to welcome and game over screens.

These improvements will make the game more engaging, user-friendly, and visually appealing.