Install kwin-decoration-oxygen by entering the following commands in the terminal:
sudo apt update sudo apt install kwin-decoration-oxygen
Description:
KWin decoration for the Oxygen desktop theme
The Oxygen desktop theme is used by Plasma and other KDE Software. . This package contains the KWin decoration and settings modules.
Homepage: https://projects.kde.org/projects/kde/workspace/oxygen
Version: 4:5.12.4-0ubuntu1
Section: universe/kde