[FR] Implement a dark mode using standard, global colors

[aminomancer]

We should add a dark mode for prefers-color-scheme: dark. Something neat I found is that you can prevent dark reader from automatically darkening a site by adding <meta name="darkreader-lock">.

It'll be easier to do this if we use global colors across the app. There's a lot of duplication between components as they all have their own stylesheets. I'd prefer if we used CSS3 custom properties for these values instead of hard-coded values or SASS variables. Then we can just implement the dark mode with a single rule that sets all the color properties to their dark mode values - the same way Firefox handles color scheme/forced color modes.