1. An extensible schema stored as computer-executable instructions on a computer readable medium for defining a respective visual appearance for a plurality of computer system components in accordance with a user interface skin theme, the schema comprising: a system schema file containing skin-theme metadata that describes allowable form and content of theme-authoring data that defines at least one display attribute for at least one user interface component, the system schema file including at least one definition of a plurality of user interface parts, wherein a theme packager uses the system schema file for validating contents of a theme-authoring file, and the system schema file including a definition of a plurality of user interface part states corresponding to at least a respective one of the user interface parts.