Add dark mode functionality

[virejdasani]

https://github.com/ankitsaxena21/Food-Ordering-App/issues/19

This is what it looks like:

• Light Mode

• Dark Mode

• Let me know if you need any more changes

• Also, I used Prettier as the code formatter for prettier code :)

[ankitsaxena21]

LGTM.

thank you for contributing to this project @virejdasani.

[virejdasani]

Thanks!