Add Dark/Light Theme Toggle with Persistent User Preference

[rahana-19]

Currently, the Memory Block Game uses a single visual theme. Many users prefer switching between Dark Mode and Light Mode, especially for better visibility and reduced eye strain.

This feature will introduce a theme toggle button that allows users to switch between dark and light themes. The selected theme should persist even after refreshing the page.

Acceptance Criteria
A toggle button (🌙 / ☀️) is visible in the header
Clicking the toggle switches between:
Light Theme (default)
Dark Theme
Selected theme is saved in local Storage
After page refresh, previously selected theme remains active
Smooth transition animation between themes

Why This is Useful
Improves user experience
Reduces eye strain during long gameplay
Makes the UI feel modern and professional
Adds personalization feature
Beginner-friendly enhancement for contributors

[FOSSCLUB-LBSITW]

Hi 👋
I noticed that a Dark/Light theme toggle feature was previously implemented in an earlier issue.
This issue seems to describe a similar feature with persistence using localStorage.
Could you please clarify whether this is meant to enhance the existing implementation (e.g., adding animations or UI improvements) or if something is missing in the current version?
Just checking to avoid duplication. Thank you!