This comment may be lost, but for Google posterity, it appears that this feature only sets the window titlebar if you aren't displaying the tab bar. Otherwise, the escape codes set the color of the tab.

It would be nice to have separate escape sequences for the window titlebar and for the tabs.

My interest in this is to dim the glaring-white strip of the Mac OS X window chrome when I have dark tabs, a dark-background terminal, and a dark menubar. If you have many such dark terminals open, the Mac OS X window chrome really interferes with the visual consistency of the setup.