A system theme schema file format includes declarations of enumerations, properties, control parts, and control part states for providing extensible theming of the visual appearance of a computer operating system's user interface components. Custom theme schema files can extend the system theme schema by adding declarations of custom enumerations, custom properties, custom control part, and custom control part states, thereby participating in theming of the visual appearance of components of the computer's operating system. The schema files establish the allowable form and content of data that specifies how user interface components should be displayed in accordance with particular themes. Themes are defined and user interface components are displayed by the operating system in accordance with the information specified in the schema files.