Comment # 11 on bug 1191402 from Fabian Vogt

This sounds like GTK is (again) ignoring Plasma settings.

Is xsettingsd running? Is xdg-desktop-portal-kde installed and running?
xdg-desktop-portal-gnome should not be installed or running.

What's the output of:

qdbus-qt5 org.kde.kded5 /modules/gtkconfig
qdbus-qt5 org.kde.GtkConfig /GtkConfig org.kde.GtkConfig.gtkTheme