@NonNullApi @NonNullFields
See: Description
| Interface | Description | 
|---|---|
| HierarchicalThemeSource | Sub-interface of ThemeSource to be implemented by objects that
 can resolve theme messages hierarchically. | 
| Theme | A Theme can resolve theme-specific messages, codes, file paths, etcetera
 (e.g. CSS and image files in a web environment). | 
| ThemeSource | Interface to be implemented by objects that can resolve  Themes. |