Add dark mode CSS

[morgant]

Not a high priority, but now that most modern browsers support dark mode CSS with the prefers-color-scheme media query, it might be nice to offer such styling.

[splorp]

Great idea.

[morgant]

I've implemented a first pass, though not deployed. I wasn't able to keep our blue link color (which matches the headings and Newton logo colors) due to contrast issues, but I think it looks good enough. I tried to dial back contrast slightly so it's not as harsh reading, while still maintaining color blind accessibility (though I haven't fully tested yet).

[splorp]

A nice option related to this would be to offer a manual style switcher, maybe as a link in the footer?