To provide theme for flutter app, apply theme in MaterialApp widget. theme needs ThemeData widget to set theme and useMaterial3, colorScheme and appBarTheme, and so on. When user slide left or right to delete item, red background is convenient to see because it lets user know, it will be deleted, so to apply this, in Dismissible widget, you need to apply background with Container widget. For color attribution, you need to provide Theme.of(context).colorScheme.error.