Install gnome-themes-extra by entering the following commands in the terminal:
sudo apt update sudo apt install gnome-themes-extra
Description:
Adwaita GTK+ 2 theme — engine
This is the version of Adwaita, the standard GNOME theme, for the GTK+ 2.x toolkit. . The GTK+ 3.x version is already included in libgtk-3-0. . This package contains the Adwaita theme engine.
Version: 3.28-1ubuntu1
Section: gnome