feat(UI): create a dark mode toggle

[a2937]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

Closes #828

I updated the CSS so that it has a toggle for dark mode. The only thing I'm not quite satisfied with is that there is no indicator on the button to determine whether or not the website is already in dark mode. However it is quite apparent in the website itself.

[github-actions]

This is a quick reminder that we can not accept pull requests from forks (including staff and prior contributors).

This repo is a "special" case because of how we build, test and deploy the codebase. If you are working on something, Please open a new pull request from a branch on this repository.

[raisedadead]

Hey @a2937!

I noticed you worked on this. And it was closed by the bot. Let me confirm with @ahmaxed if this is something we want in the first place, if we do and he confirms you should be able to push your branch to this repo (upstream) directly and recreate the PR.

Thanks.

[ahmaxed]

Thanks for working on this feature, @a2937.
I will update the issue with further spec and look forward to seeing this feature on news.

[a2937]

Thank you. I appreciate being given this wonderful opportunity.