Theme toggle button

## Overview
**TL;DR**

We should include same theme toggler, that we use in github for this website.
### Extra Detail

#### Screenshots


#### Reasoning

### How to Contribute
- [x] Comment on this issue if you'd like to work/collaborate on it!
- [x] Fork the [repository](https://github.com/github/training-kit).
- [x] Use the [GitHub Flow](https://guides.github.com/introduction/flow/) to make changes to your fork. [This](https://lab.github.com/githubtraining/introduction-to-github) is a refresher course if you're unsure about how to make a change on GitHub.
- [x] Push your changes to your repository.
- [ ] Submit a Pull Request
- Base Dropdown: github/training-kit
- Compare Dropdown: Your fork

#### Testing Locally
If you'd like to make and test changes locally (and see how they would look if merged), do the following:
- In your command line:
- [Install Git](https://git-scm.com/)
- [Install Ruby](https://www.ruby-lang.org/en/documentation/installation/)
- [ ] Run `script/setup`
- [ ] Run `script/server`.
- When successful, the script will initiate a local server at `http://127.0.0.1:4000/`.

### Questions?
- Leave a comment on this issue! Make sure to use `@mentions` if you want a specific person's attention!