CSS dark theme for HLJS

[david-christiansen]

Once the imminent merge of #383 is completed, we'll have a nice dark mode. However, the HLJS theme (used for styling code samples) should be either built to match the Hackage colors that we use or selected to look good in the dark. Right now, it's a bit of a stop-gap.

It would be great to:

• [ ] Design/find a code-highlighting theme (perhaps we can share this with haskell.org and Haddock?) for light mode rather than getting one off the shelf
• [ ] Design or find a theme for dark mode that also matches other Haskell web branding
• [ ] Arrange for it to be swapped in the CSS along with the other colors