The Appearance Manager is a component of Mac OS 8 and Mac OS 9 that controls the overall look of the Macintosh graphical user interface widgets and supports several themes. It was originally developed for Apple's ill-fated Copland project, but with the cancellation of this project the system was moved into newer versions of the Mac OS. The Appearance Manager is also available free as part of a downloadable SDK for System 7.
The Appearance Manager is implemented as an abstraction layer between the Control Manager and QuickDraw. Previously, controls made direct QuickDraw calls to draw user interface elements such as buttons, scrollbars, window title bars, etc. With the Appearance Manager, these elements are abstracted into a series of APIs that draw the item as a distinct entity on behalf of the client code, thus relieving the Control Manager of the task. This extra level of indirection allows the system to support the concept of switchable "themes", since client code simply requests the image of an interface element (a button or scroll bar, for example) and draws its appearance. Kaleidoscope, a 3rd party application, was the first to utilize this functionality with via "scheme" files, followed by an updated Appearance Control Panel in Mac OS 8.5, which acted similarly via "theme" files. Schemes and themes are similar in concept, but they are not internally compatible.
An updated and more powerful version of the Appearance Manager was used for Carbon applications in Mac OS X even after Apple made the transition to Aqua. The Extras.rsrc file is an updated version of an Appearance Theme that is compatible with the newer Appearance Manager. As of Mac OS X version 10.3, 'layo' data is no longer used, even for Carbon applications, so the continued existence of the Appearance Manager can no longer be confirmed.