Add dark mode

[CompSciLauren]

Is your feature request related to a problem? Please describe.
There is only one theme available for the website currently and it is a light mode theme. It would be nice to have a dark mode option for users who prefer to use dark mode or happen to be looking at the website in some setting that would be better with dark mode.

Describe the solution you'd like
Add a dark mode option that would apply to every page on the website.

Additional context
It will be important to consider how to actually implement dark mode in a way that remains accessible and appealing to users. We will want to refer to best practices for this rather than trying to come up with the exact colors and shading on our own.

Good article on dark mode: https://css-tricks.com/a-complete-guide-to-dark-mode-on-the-web/
Terra themes: https://terra-dev-si-collapsing-bn3drj.herokuapp.com/secondary-nav-test/terra-dev-site/themed