Install breeze-gtk-theme by entering the following commands in the terminal:
sudo apt update sudo apt install breeze-gtk-theme
Description:
GTK theme built to match KDE's Breeze
A GTK Theme Built to Match KDE's Breeze. Widget theme for GTK 2 and 3.
Homepage: https://projects.kde.org/projects/kde/workspace/breeze-gtk
Version: 5.12.4-0ubuntu1
Section: universe/kde