Visually recognise active window

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http://askubuntu.com – I work with two monitors, usually with lots of different windows open. I don't like to use the mouse, so swap between applications using alt-tab usually. The problem comes when I want to type something into an application. I need the active application to immediately obvious, usually so that I can tell which monitor to look at next. With default ubuntu (with appearance = ambiance), the only real visual indication of a window being active is that the header bar of the application is in a lighter font color. This is too subtle for me, so I find myself alt-tabbing and moving my mouse too much (HowTos)