from gi.repository import Gtk

def on_button_clicked(button, tv):
    style = tv.get_style_context()
    bgcolor = style.get_background_color(Gtk.StateType.NORMAL) 
    print(bgcolor) 

win = Gtk.Window()
vb = Gtk.VBox()
tv = Gtk.TextView()
vb.add(tv)
button = Gtk.Button('get bg')
vb.add(button)
button.connect('clicked', on_button_clicked, tv)

win.add(vb)
win.show_all()

win.connect('delete-event', Gtk.main_quit)

Gtk.main()