"Adwaita-dark" is supplied by gnome-themes-standard and has a distinct
name to theme GTK+ 2. For GTK+ 3, this theme just imports gtk-contained-
dark.css from the library, so it's _as if_ Adwaita was the system theme.

But Nautilus only applied its Adwaita-specific CSS if the system theme
name was exactly "Adwaita". So using "Adwaita-dark" meant we got Adwaita
but without Nautilus' additions to it, even though they are applicable.

Fix this by also loading Adwaita.css if the theme name is "Adwaita-dark"

https://bugzilla.gnome.org/show_bug.cgi?id=779266