Implement Theme Toggle (Dark ↔ Light Mode) with Persistence

[Zunairah-k]

Issue Description:

Right now, the site is stuck in dark mode forever.
Let’s make it smarter by adding a theme toggle feature so users can switch between dark and light modes.

What needs to happen:

• Set up CSS variables for both light and dark themes.

• Add a React state to manage the current theme.

• Add a toggle button (🌞/🌙) in the navbar for switching themes.

• Save user’s theme preference in localStorage so it stays even after reload.

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[Zunairah-k]

@Avdhesh-Varshney id like to work on this, pls assign me the issue