Themer for Mac

From AquaWorld:

Themer is designed to make switching themes on Mac OS X easy. Its Cocoa interface is designed around ease-of-use and clarity, and yet it still supports all the features and more of other, more difficult to use, theme changers. It takes the new standard theme format TPKG2 (.dlta) and also just raw resource files (rsrc). In addition to this you can also use the icon set format with which you can have complete control over the icons used by your computer. You can preview themes before installing them, as well as theme specific applications (in contrast to the entire Operating System).
The primary focus of Themer v3, however, has been safety. Themer now uses the most comprehensive safety system of any theme changer available. The combination of the Consecutive Backup Mechanism and all new SafeTheme annihilates all of the old problems that users had with all the theme changers.
The "Application Only" download contains only the Themer application. "Starter-Kit 1" contains 1 theme and 1 icon set. "Starter-Kit 2" contains 4 themes and 1 icon set.