gnome-themes-extra-data 3.28-2ubuntu1 (amd64 binary) in ubuntu lunar

 This package contains the common architecture-independent files for
 Adwaita, the standard GNOME theme, for the GTK 2 toolkit. The GTK 3
 equivalent is already included in libgtk-3-0.
 .
 This package also makes the dark variant of Adwaita available for GTK
 3 as a separate Adwaita-dark theme. In newer versions of GTK 3, this
 has been superseded by the ability for a single theme to provide both
 light and dark variants, which can be selected in the Appearance panel
 of GNOME's Settings application (gnome-control-center).