KDE Plasma KDialog問題
I was facing the exact same problem, here is how I solved the problem.
Go to System settings -> Application Style -> GNOME Application Style (GTK)
"Select a GTK2 Themes" AND "Select a GTK3 Theme" both use "Breeze-Dark"
Problem solved! Apparently, there is a conflict between Ubuntu MATE (GTK2) and KDE (GTK3)