css: add dark color-scheme

This doesn't work so well for blogs that hard-code the background-color, but not the foreground-color for some elements. But this isn't really something people should do, so let's instead leave that as a problem for the blog authors to fix. The only example I've seen of this, is this recent post:

https://blogs.gnome.org/uraeus/2022/06/10/how-to-get-your-application-to-show-up-in-gnome-software/