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 the system schema file comprises: (1) a theme packager for generating a package theme file based on system theme schema, defined custom theme schema, and a theme authoring file, wherein the theme packager validates the contents of the theme authoring file based on the contents of the system schema file and the contents of the defined custom theme schema; and (2) a theme manager for re-validating the contents of a theme authoring file; 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; and the theme authoring file comprising at least one custom theme schema file for extending the system theme schema by adding at least one of: declarations of custom enumerations, custom properties, custom control parts, and custom control part states.