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Themes are packages containing specific graphical appearance details and are meant to modify the appearance of an operating system.
OS themes are mostly used to customize the interface, rather than add any new functionality, so they will even modify the desktop shortcuts for various applications.
Themes also include sounds that will replace the default ones from the host computer (the original sound scheme can be easily restored once the theme is uninstalled).
There are also specialized applications that can transform the appearance of an operating system to resemble another, for instance XP can be made to look like Vista, Windows Seven or MAC OS.