Install gtk3-engines-breeze by entering the following commands in the terminal:
sudo apt update sudo apt install gtk3-engines-breeze
Description:
Transitional package for KDE's Breeze
This transitional package allows one to migrate to the package providing the KDE's Breeze GTK theme. . It can be safely removed after the installation.
Homepage: https://projects.kde.org/projects/kde/workspace/breeze-gtk
Version: 5.12.4-0ubuntu1
Section: universe/kde