XRevert (formerly YosemiteRevert), as the name suggests, changes the icons in OS X Yosemite (10.10) and later back to the icons from earlier OS X versions. This includes icons for all built-in apps, menu bar icons such as the battery icon, Finder icons such as external drive icons and the default folder icon, and even the icons for preference panes in System Preferences. Now, you can enjoy all of the amazing new features in OS X Yosemite without losing the 3D look and feel of the operating system.
Mac OS X 10.10,
Mac OS X 10.11