I have been following the documentation

Thanks in advance for any pointers or tips!

CodePudding user response：

I think it is fetching colour from scaffold. You can try

scaffoldBackgroundColor : Colors.red,

Not sure if you can change the text using the sign in screen widget. If you wish to customise the widgets please take a look at AuthFlowBuilder

Reference https://firebase.flutter.dev/docs/ui/auth/building-a-custom-ui/