Install gtk2-engines-qtcurve by entering the following commands in the terminal:
sudo apt update sudo apt install gtk2-engines-qtcurve
Description:
QtCurve widget style for applications based on GTK+ 2.x
This package contains the QtCurve theme engine for GTK+ 2.x. . This package is most useful when installed together with kde-style-qtcurve. . QtCurve is a set of widget styles for Qt and GTK+ libraries. It provides a consistent look between KDE, GNOME and other applications based on these libraries, which is easy on the eyes and visually pleasing.
Homepage: https://cgit.kde.org/qtcurve.git
Version: 1.8.18+git20160320-3d8622c-5build4
Section: universe/gnome