Adjust unauthenticated flows to phone dark mode

[uiboy]

ios now supports a dark mode , android already has had this for a while.

We should be able to read this and change our splash screen and registration flows to darker tones.
For material app, flutter supports a dark theme like this

MaterialApp(
  theme: ThemeData(
    brightness: Brightness.light,
    primaryColor: Colors.red,
  ),
  darkTheme: ThemeData(
    brightness: Brightness.dark,
  ),
);

Theres some ongoing work around some Cupertino widgets though
flutter/flutter#39685
flutter/flutter#33873

1. Would require us to set dark/light theme across all the steps of the registration / forgot password / sign up / login flows.

2. Use a dark splash image on the splash screen.

3. Also the default theme when logging into the app should be a dark one if phone is in dark mode and there is no stored theme in the storage.