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There is no way to do it, and I doubt Apple will make a way to do it. That being said, I've heard that some theme hackers are already working on ways to do it - along with changing the white-isn transparent windows to the black windows. Who knows if it'll ever happen, but it would be cool!
 
Well, you can adjust it - only between two options though.

If you go to Sys Prefs/Accessibility/Display, there is an option to reduce transparency.
 
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