Assuming your GTK2 (/etc/gtk-2.0/gtkrc) and GTK3 (/etc/gtk-3.0/settings.ini) settings files are correctly set as you want. Create the next symlinks and change the ownership:
sudo ln -s /etc/gtk-2.0/gtkrc /usr/share/gtk-2.0/gtkrc
sudo ln -s /etc/gtk-2.0/gtkrc /usr/share/gtk-2.0/gtkrc ~/.gtkrc-2.0
sudo chown username:username .gtkrc-2.0
sudo ln -s /etc/gtk-3.0/settings.ini ~/.config/gtk-3.0/settings.ini
sudo chown username:username ~/.config/gtk-3.0/settings.ini
Please note that you have to delete the files before creating the symlinks. Thus, you create a symbolic link in your home user directory to the system one (/etc/gtk-2.0/gtkrc
and/or /etc/gtk-3.0/settings.ini
).