Transparency on Focused/Unfocused windows

Inst开发者_StackOverflow中文版ead of using borders, I want to set the alpha of focused and unfocused windows to, say, 90% and 70%. This would apply to all windows except those matching some criterion like className = "Gimp". How can I implement this in my xmonad?

Thanks!

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You can use XMonad.Hooks.FadeInactive. Note that it requires Xcompmgr or something of a kind.