>>> How to set Qt application to Gtk2 style

apt-get install qt4-qtconfig

Run qtconfig-qt4
Select "Gtk+" from "Select GUI Srtyle" menu
Then from the menu select :
File -> Save
File -> Exit

The configuration files are ~/.config/Trolltech.conf (qt) and ~/.gtkrc-2.0 (gtk) .

If you want root to use the same theme (if you run an application as root via gksu [synaptic]), copy the configuration files to /root

sudo mkdir /root/.config
sudo cp ~your_user_name/.config/Trolltech.conf /root/.config/
sudo cp ~your_user_name/.gtkrc-2.0 /root/