Theme widget takes a

State management

State is information that can be read synchronously
when a widget is built or information
that might change during the lifetime of a widget.
To manage app state in Flutter,
use a StatefulWidget paired with a State object.

For more information on ways to approach managing state in Flutter,
see State management.

The StatelessWidget

A StatelessWidget in Flutter is a widget
that doesn’t require a state change—
it has no internal state to manage.

Stateless widgets are useful when the part of the user interface
you are describing does not depend on anything other than the
configuration information in the object itself and the
BuildContext in which the widget is inflated.

AboutDialog, CircleAvatar, and Text are examples
of stateless widgets that subclass StatelessWidget.