Added dark mode toggle

[Pranayee35]

Added a Dark Mode toggle button to enhance the user interface.

The button allows users to switch between Light Mode and Dark Mode.

When Dark Mode is activated:

 -The page background becomes dark and text turns light.

 -The button text automatically changes to "Light Mode".

When Light Mode is activated:

 -The page returns to its default colors.

 -The button text switches back to "Dark Mode".