Want to set different Theme for a particular widget

Just wrap the widget with the Theme Widget and pass the ThemeData().

Theme(
  data: ThemeData(...),
  child: TextFormField(
    decoration: const InputDecoration(
      icon: Icon(Icons.person),
      hintText: 'What do people call you ?',
      labelText: 'Name *',
    )
    validator: (value) {
      return value!.contains('@') ? 'Do not use the @ char' : null;
    }
  )
)