[Feature] Unlockable Themes Based on Score Milestones

📝### **_Description_**

Introduce a feature that allows players to unlock different visual themes for the game as they accumulate points through gameplay. These themes will change the game's background, paddle, ball colors, offering visual variety and gamified motivation.

_**### 🎯 Purpose:**_

This enhancement introduces a reward system that increases user engagement, making the game more interactive and giving players long-term goals. It also adds a layer of customization and progression, similar to popular games like Temple Run and Subway Surfers

_**### 🌟 Proposed Enhancement:**_

1. Add 2 themes (e.g., Space Warp, Snow Freeze ).
2. Each theme unlocks when the player crosses a specific score milestone (e.g., 50,100).
3. Use localStorage to save unlocked themes across sessions.
4. Create a theme selection modal (popup) that displays:
- Locked/Unlocked status of themes.
- Preview of each theme.
- Option to select currently unlocked themes.
- Locked/Unlocked status of themes.

**_Suggested Labels:_**
gssoc25, Level 3, Feature, UI, Game Logic, Good First Issue