If you start a GTK GUI with root permissions and you notice that it uses the old GTK theme, you can make all applications started as root to use your current theme by doing this:
sudo ln -s ~/.themes /root/.themes
sudo ln -s ~/.icons /root/.icons
sudo ln -s ~/.fonts /root/.fonts
This will basically create symlinks to your icons, fonts and themes directories that draw your current GTK theme.
From Tips4Linux.com