The release of OS X Yosemite in 2014 gave the Mac's user interface a fresh coat of paint in part through adopting much of the transparency effects that Apple added to iOS 7 in prior year.
Transparency is used throughout macOS and can be observed on the desktop and seen in drop-down menus, title bars, sidebars and toolbars.
This post will teach you how to disable transparency for all user interface elements that have a translucent appearance on your Mac.