This uses `prefers-color-scheme: dark` to automatically switch
to dark mode if the user specifies it in their system settings.
Dark mode is supported in Firefox 67, Chrome 76, Safari 12.1,
and iOS 13 as of this commit, but the current status can be
checked here: https://caniuse.com/#feat=prefers-color-scheme
Additionally, this uses CSS Variables to implement the color-switching
mechanism. This isn’t supported in IE, but the site degrades
reasonably well with them disabled.
Support table: https://caniuse.com/#feat=css-variables
