ConstDefaults to 'light': a component embedded in a page cannot know whether the page follows the OS, and
most pages are light, so it stays light unless the host says otherwise. An app shell that owns the whole
surface (Studio) provides 'system'. Provided once per tree so every layer that resolves the mode
independently (workbook, sheet, charts, floats) agrees.
The ThemeModeFallback for every useResolvedThemeMode below it that does not pass its own.