Thanks to Aleix Pol, Manuel Tortosa and José Antonio Sánchez, there is now a tool for easy configuration of GTK2 and GTK3 look under KDE:
There are deb-packages for non-netrunner-distributions (which work for many deb-based KDE-distros like Kubuntu, MintKDE, etc.):
Just click on the link named “kde-gtk-config(_version)” of your version (precise, quantal, raring), then choose your architecture Package file (amd64, i386).
After installation, you will find the configuration module under “System Settings – Application Appearance – Gtk Configuration”.
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