This button. Click it.
Seriously. Transparency is bullcrap. No, Sir Jony, your OS is not all about "focusing on my content", you're just smearing crap all over the screen. When I have a window open, I want to see that window, not part of what's behind it. You're making what I want to read, harder to read. I want to read what I want to read, not what's behind it. How is that so hard to understand?
I worked for a publisher. Like almost all publishers, we used the cheapest paper possible, but we weren't so cheap as to use paper that you could see through. That makes things worse, not better. Didn't you guys move away from this once already?
I can't wait for brushed metal to come back in OS X 10.11: Pittsburg. (Coincidentally, there is a Pittsburg in California, so that name could work.)