I have Kubuntu 16.04 with kde 5.6.5, and I cannot change the graphical components, I am stuck with the qtcurve style.
I am using a laptop with intel graphics and nvidia card, and I had problems insalling drivers because of the optimus system.
While trying to reinstall my graphics drivers, I ended up installing the ubuntu desktop on my system (I don't remember the exact command).
On the login screen, I can now choose between plasma and Ubuntu.
I don't know if it's related, but I thought it was useful to mention it.
I tried rebooting, no luck.
This is not an urgent problem (qtcurve style is good), but this is annoying and I like having a working system 😀
Any help is welcome.