Dark mode styling for the website

Inspired by Dago A Carralero's email earlier today, I started looking into
what it would take to actually add a dark mode to the entire website. Turns
out it's not actually that difficult; the attached patch took just a couple
of hours of fiddling.

It leaves the existing styling as the default, and uses CSS media queries
to override certain styles if the browser reports that dark mode is
preferred. Colours are loosely based on the dark theme for pgAdmin (which
is of course, based on PostgreSQL blue), with a few other new ones added
where there were no suitable ones in the pgAdmin palette.

Of course, this is a PoC at this point, for review and comments. I may have
missed some styles, and the actual colors used are obviously a matter for
some serious bikeshedding.

Thoughts, comments, and paint colour suggestions welcome.

--
Dave Page
Blog: https://pgsnake.blogspot.com
Twitter: @pgsnake

EDB: https://www.enterprisedb.com