Widgetbook allows developers to define whatever Theme they have defined for their app. To accompany every theme and the ones defined by Flutter, the following constructors exist:

ThemeConstructorDefaults to
Custom ThemeWidgetbook<YourTheme>

As you can see from the constructor definitions, Widgetbook allows to define your Theme type to accompany any implementation. However, most developers will likely use Widgetbook.material as shown here.