Preview transparent BG hack?

[interweb]

Howdy,
I'm interested if there's a hack to make Preview's background color transparent... Terminal and other programs like TextMate allow for a clear background.

It seems that Preview allows us to only change the color and not the opacity. Can anyone figure this out??
There's got to be a Terminal hack to turn on this feature...

 

[interweb]

Afloat

So, after a bit more research, there is a nifty App called Afloat.
http://infinite-labs.net/afloat/
But, I'm wondering if there is something that's a bit more simple. Afloat has some great options. Specifically with transparency, it makes the entire window transparent, not just the background—keeping the interface elements and title-bar—like Terminal or TextMatte...

If Preview allows one to change the background color (there's even a non-functional transparency slider) - shouldn't you be able to see application windows and/or the desktop through the background?