Added Theme Selector Feature to Pixel Love Letter Generator

[Hirannya-Mhaisbadwe]

✨ Feature: Theme Selector for Love Letter

💡 Summary

Added a theme selector dropdown that lets users switch between multiple love letter themes (e.g., Classic, Romantic Rose, Midnight Blue, Pixel Pink).

🔧 Changes

• Updated index.html to include a theme selector dropdown.
• Added dynamic theme switching logic in script.js.
• Created new theme styles in style.css with smooth transitions.

❤️ How It Works

Users can choose a theme from the dropdown menu, and the letter preview updates instantly with the selected style.

📸 Future Enhancements

• Save user’s last selected theme in local storage.
• Add more themes for Valentine’s edition 💘.

Closes #251

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[Hirannya-Mhaisbadwe]

Note: The Vercel check failed because this branch is from a fork. The code runs fine locally.