Add dark and light mode toggle with persistent theme preference

[AditiBS]

📌 Description

This pull request introduces a Dark / Light Mode toggle to the Memory Block Game, improving accessibility and user comfort across different lighting conditions.

Previously, the application supported only a single theme. With this update, users can now switch themes based on their preference

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🔗 Related Issue

Closes: #70

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🛠 Changes Made

• Added a theme toggle button (Dark / Light mode) in the Game Settings panel
• Implemented CSS variables to manage theme colors efficiently
• Applied the selected theme consistently across the entire application UI

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📷 Screenshots (if applicable)

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✅ Checklist

• I have tested my changes
• My code follows project guidelines
• I have linked the related issue