1. Open nautilus as root
gksudo nautilus
Navigate to /home/username/.themes and copy all of them to /usr/share/themes
(.themes is a hidden folder, press “ctrl+h” to view them)
2. Change the gdm behaviour
In a terminal copy/paste this
sudo gedit /etc/gdm/gdm.conf
Look for [gui] and make sure these lines look like this:
GtkTheme=all
GtkThemesToAllow=all
If there is a # before them, delete it.
Now synaptic and other root applications should use the gtk theme you are using on your desktop.