It should be possible to choose between at least two "titlebar themes". One, the default, would be using the standard cocoa design as iTerm2 does already, and a second would use a black/dark titlebar design instead to fit better with dark iTerm2 color themes.

An example of an application that uses a black titlebar is Spotify (screenshot attached), which has an all-over black design which makes the black titlebar necessary. An app like iTerm2 also uses a black theme as default and many keeps using a black theme for iTerm2 so it would be fitting to have a dark/black titlebar too as to not look so weird and out of place with the dark El Capitan menubar in OS X, light titlebar on the iTerm2 window and dark theme inside iTerm2 as can be seen in the second screenshot I have attached.