Now that the screen shield is gone (at least, as it used to
be), the corresponding session mode is not necessary anymore
as well.
Remove the 'lock-screen' session mode, and the corresponding
CSS.
https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/872
- 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