GTK 2.0 is quite old now. (GTK 4.0 is going to be released soon.) You should move to at least GTK+3.x
In GTK3.x the way to do it is through CSS, as nar6du14 has mentioned.
I've posted a tutorial in the section "Gtk+ Tutorials & Resources"http://www.gtkforums.com/viewtopic.php?f=3&t=988&p=72417=Styling+GtkTreeView#p72417
For a full list of style-able properties click "Style Properties" in the upper right of the GTK manual page.
The following link will always give you the current stable version. Although styling for GtkTreeView hasn't changed through all of the GTK3.x series I would suggest always using the GTK manual for YOUR version because there are some differences among other widgets.https://developer.gnome.org/gtk3/stable/GtkTreeView.html