The problem is that I cannot integrate GTK applications in KDE (Kubuntu 13.04).
My version of KDE is 4.11, from kubuntu-backports. The same exact problem appears in 4.10.5, from raring-updates.
I recently reinstalled the system and removed my home folder, to get rid of old thing that I was carrying with me since too much time, so no previous settings are involved into this.
The GTK application I use most is Chrome (updated flash + sync of data with smartphone, + other things), so I can workaround with a specific theme for it. But also Gparted is GTK, as bumblebee-config, and many others.
At this moment, the GTK applications have squared buttons, like very old-style Gnome applications (or Win98, for people not familiar with Gnome) and the colours in them are absolutely "random" (violet background when selecting text, dark blue in menus).
Before reinstalling and removing data from the home folder, GTK applications were well-integrated with KDE and oxygen themes.
Any idea? :)