Step2-03 exercise 'customize default button with a styles function'

Step2-03 exercise 'Customizing one Fabric control instance, 4. Try to customize this with a styles function' could not implement. Step02-3 demo TodoHeader.tsx 'TextField' could implement this, because styles property of TextField is IStyleFunctionOrObject type. But in Step2-03 exercise DefaultButton's styles property is IButtonStyles type, which cannnot use function.