Automatically adjust colour scheme for "dark mode".

Automatically adjust colour scheme for "dark mode".

Playing around a bit, let's try to respect "dark" mode in firefox and
support automatically switching colour scheme accordingly.

We convert all colour references in the stylesheet to new SASS mixins
that will generate rules to adapt to the correct colour scheme.  The
selection is done using CSS variables that should gracefully degrade
to the original (light) colours.