The inactive state is the NORMAL state. From the Python gtk docs
gtk.Style holds information for the five possible widget states
though not every widget supports all five states:
gtk.STATE_NORMAL The state of a sensitive widget that is not active
and does not have the focus
gtk.STATE_ACTIVE The state of a sensitive widget when it is active
e.g. a button that is pressed but not yet released
gtk.STATE_PRELIGHT The state of a sensitive widget that has the focus
e.g. a button that has the mouse pointer over it.
gtk.STATE_SELECTED The state of a widget that is selected e.g.
selected text in a gtk.Entry widget
gtk.STATE_INSENSITIVE The state of a widget that is insensitive and
will not respond to any events e.g. cannot be activated, selected or
I've done some GTK2 programming in Python over the last few years, but I'm certainly not an expert in all of its intricacies. And I haven't played around much with themes. I'm not sure if what you want to do can be done, but hopefully someone with more expertise will be able to make a definitive statement in this regard.
FWIW, I do know how to make a slider in an GTK application change color depending on whether the main window currently has focus or not, but I guess that's not much use to you. But if you want to play around with it, let me know & I'll paste my Python code into this answer.