Install gtk2-engines-wonderland by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-wonderland
Description:
Wonderland theme for GTK+-2.0
Original Bluecurve engine from Red Hat's artwork package. . This package is based on the Wonderland theme available from http://art.gnome.org/ for GTK+-2.0. This theme is not available for GTK-1.2 at the same location and thus is not provided by this package.
Homepage: http://art.gnome.org/themes/gtk2/274
Version: 1.0-11
Section: universe/x11