No exercicio dão a tarefa de alinhar blocos em um fundo branco, mas falta a parte do Container branco, deve ficar assim:
Stack(
alignment: AlignmentDirectional.center,
children: [
Container(color: Colors.white),
Container(height: 300, width: 300, color: Colors.black),
Container(height: 150, width: 150, color: Colors.red),
Container(height: 75, width: 75, color: Colors.blue),
Container(height: 37, width: 37, color: Colors.yellow),
],
),