1. An extensible schema stored as computer-executable instructions in 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 for specifying allowable form and content of 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, 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.