As you know, by using System Settings as your own user, you can set your GTK applications up (via GTK+ Appearance in the Appearance section) to use the QtCurve widget style, making them assume an Qt4-ish look. But the setting is only set for your user and doesn't apply to root.
One thing you can do is to open up System Settings as root and clone your settings, particularly ones relating to appearance (including GTK+ Appearance). 'kdesudo systemsettings' in a run box (Alt+F2) or a terminal.
You can also make symbolic links in the home directory of root, to make it use your user's setting. The files touched are ~/.kde/share/config/kdeglobals and ~/.gtkrc-2.0-kde4.
The second solution makes root always follow your user's settings, which can be a good or a bad thing. If you use this approach, then even programs started as root will be updated to use new themes and colors if you change them. If you use the first approach, you'd have to manually make sure the new settings match.
$ sudo ln -s ~/.gtkrc-2.0-kde4 /root/.gtkrc-2.0-kde4
$ sudo rm /root/.kde/share/config/kdeglobals
$ sudo ln -s ~/.kde/share/config/kdeglobals /root/.kde/share/config/kdeglobals