Adding dark and light mode toggle button in ExpenseFlow

[Sha-mbhavi]

Adding Dark / Light Mode toggle button
User can change to dark or light mode according to their convenience through dark and light mode toggle button

[github-actions]

Thank you for raising this issue!

We'll review it as soon as possible. We truly appreciate your contributions! ✨

Meanwhile make sure you've visited the README.md, CONTRIBUTING.md, and CODE_OF_CONDUCT.md before creating a PR for this. Also, please do NOT create a PR until this issue has been assigned to you. 😊

[Sha-mbhavi]

@Renu-code123 Please assign me this issue.(ecwoc)