* Update Gemfile.lock
Running Jekyll according to `CONTRIBUTING.md` fails complaining about
missing a `nokogiri` dependency. Updating `Gemfile.lock` seems to solve
this issue.
Fixes: #500
* Website: Support dark mode
Support `prefers-color-scheme: dark` by employing some more or less
hacky CSS overrides.
One should perhaps just use a different off-the-shelf Jekyll theme that
does this properly from the start.