I am using Kubuntu 14.10 and it's working quite well so far, but my problem is pretty much what it's stated in the title, when executing a GTK app that requires sudo privileges, such as Synaptic package manager, it's displayed with an ugly theme and huge fonts. However if starting the app without sudo, oxygen-gtk theme is respected. Sometimes the app won't even run until next reboot, displaying a message in terminal about not being able to open display X.
I've tried every possible theme in "System configuration -> Application appearance -> GTK" and other fixes suggested on forums around the web and even here in askubuntu but nothing seems to work. Any suggestions?