- panel fg color was derived from bg color. Not a good idea as it's not
just light/drk, but HC as well.
- deriving from dark theme means contrast for things like popover items
is better.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/904
The sass sources now live in a project in GNOME, so they can
be used in multiple projects like gnome-shell-extensions.
Because of that, add gnome-shell-sass as a submodule of gnome-shell
and remove the sass sources from the theme directory.