Visual indication of focus state

It would be most awesome if the user could easily recognize the focus state of a terminal.
This applies to both split terminals, as well as unfocused terminals left open for later refocus.

A possible visual indication, for example, could be a shift in transparency or background, so that upon blur the now unfocused terminal will appear fainter. On regaining focus, the original appearance will be restored.

I believe such an indication of the focus state would help the user to more easily visually orient.
I for example sometimes need to look twice to see whether my focus rests with Guake or on another application open on a different monitor.