How can you build a genuinely themeable custom widget library that automatically adapts to an app's central theme, rather than hardcoding fixed colors and styles directly within each individual custom widget?

Building a genuinely themeable, professional quality custom widget library means ensuring each individual widget reads its actual colors, fonts, and other styling values from the surrounding app's central Theme rather than hardcoding fixed values directly, typically by accessing Theme.of context within the widget's build method or by defining and consuming custom ThemeExtensions for values not covered by the standard theme, ensuring that every widget within the library automatically and consistently adapts correctly whenever the overall app theme changes, including correctly supporting both light and dark mode without requiring any individual widget specific changes.

class AppCard extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);
    return Container(color: theme.cardColor, child: child);
  }
}