Allow switching to GNOME's dark theme

• In the 2020 user survey, dark theme was pretty popular.

• https://lists.autistici.org/message/20220429.033113.016974ce.en.html

• It's now possible to switch to a decent dark theme with

  gsettings set org.gnome.desktop.interface gtk-theme "Adwaita-dark"

A next step could be to study what ways for users to switch theme might already be in Tails or available elsewhere in GNOME.