Install kde-style-oxygen-qt4 by entering the following commands in the terminal:
sudo apt update sudo apt install kde-style-oxygen-qt4
Description:
Qt 4 decoration for the Oxygen desktop theme
The Oxygen desktop theme is used by Plasma and other KDE Software. . This is the version shipped with Plasma 5. This package contains the Qt 4 widget theme.
Homepage: https://projects.kde.org/projects/kde/workspace/oxygen
Version: 4:5.12.4-0ubuntu1
Section: universe/kde