Install gtk2-engines-equinox by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-equinox
Description:
Equinox GTK+ theme engine
Equinox is a theme engine for the GTK+ widget toolkit, a heavily modified derivative of the Aurora engine (version 1.40). . GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Homepage: http://gnome-look.org/content/show.php/Equinox+Gtk+Engine?content=121881
Version: 1.50-0ubuntu4
Section: universe/x11