Install gtk2-engines-oxygen by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-oxygen
Description:
Oxygen widget theme for GTK+-based applications
Oxygen-Gtk is a port of the Oxygen widget style to GTK+. . It ensures visual consistency between GTK+ and Qt-based applications running under KDE.
Homepage: https://projects.kde.org/projects/playground/artwork/oxygen-gtk
Version: 1.4.6-1ubuntu1
Section: universe/x11