Added dark mode toggle button

[Sohamnayak4]

Added a toggle switch in the navbar component, which changes the overall theme of the body to dark.
This does not change all the individual components completely.


Related to #500

[veroandreo]

Thanks for your contribution @Sohamnayak4. I just tested it locally; it doesn't look right. Quite some other changes are needed to make it look more natural. That sort of cyan color is not part of the GRASS color palette.

[Sohamnayak4]

@veroandreo This is the initial commit for the addition of dark mode. I am working on completing this following the actual color scheme. Thanks for checking and pointing out the problems.

[wenzeslaus]

My big question here is whether the main website needs a dark theme.