If you don’t like the new transparent window effects in OS X Yosemite, you can easily disable them… or at least tone them down quite a bit.
You’ll notice in the screenshot above that the sidebar is bleeding through, and the content that was partially scrolled under the title bar is bleeding through. It’s a nice effect, but can be distracting as well.
As you would imagine, you need to first open up System Preferences and then click on the Accessibility icon.
Once there, click the Reduce transparency menu check box.
You’ll notice that the background is no longer bleeding through to the sidebar or the main toolbar where the title is.
As to why I’m using a really pink background, it’s one of the built-in new Yosemite wallpapers.