Add way to change color theme on the site

With the addition of a dark mode for the website, the only way for readers to change theme would be to change their OS preferences.

It would be nice to be able to change the color theme directly on the site. Proposed ideas include a tri-state switch or button (maybe like the MDN docs?), but optimally it would be nice to have a setting for choosing dark, light, or the OS default.

See more discussion about this here.