Add Dark Mode Toggle Button

[gouthamx67]

Currently, the application does not provide an option to switch between light and dark themes.
Adding a dark mode toggle button would improve accessibility, reduce eye strain in low-light environments, and enhance overall user experience.

Expected behavior:

A toggle button that allows users to switch between light and dark mode.

The selected theme should apply across the application.

(Optional) Persist the user’s preference across sessions.