Quite simple
You need an application called: SetalphaValue k
Project description:
SetAlphaValue is extension for “Cocoa” applications and set the alphaValue of windows. In others words, this program is only aesthetic, just for fun
It should look like that:

Some quote out of the readme:
You expressly acknowledge and agree that use of the SetAlphaValue software is at your sole risk. In no event shall SetAlphaValue or Michael Parrot be liable for personal injury, loss of date or any other incidental to use SetAlphaValue software.
SetAlphaValue is a module for the Cocoa applications and set the alpha value of the windows.
I make this application for fun !
Installation
Launch the Assistant or place the SetAlphaValue module is in folder:
/Library/InputManagers/
or :
~/Library/InputManagers/
Create folder InputManagers if does not exist.
———————————————————–
Developer: Michaï ¿ ½l PARROT,
E-mail: contact@mparrot.net
Web: www.mparrot.net
———————————————————–
June 03, 2006 v2.2.2
April 30, 2006 v2.2.1
February 10, 2006 v2.2
January 07, 2006 v2.1
January 05, 2006 v2.0
September 15, 2005 v1.9
September 13, 2005 v1.8
June 04, 2005 v1.7
January 17, 2005 v1.6
January 06, 2005 v1.5
November 13, 2004 v1.0
———————————————————–
I am using it now since 2 days and it works like supposed. Unfortunally this feature is limited to Cocoa apps, but thats more then before
enjoy it
fidel