Setup the Color Theme

[Arthi-chaud]

The color scheme form the previous Flutter PoC was the following:

theme: ThemeData(
        colorScheme: const ColorScheme(
          primary: Colors.green,
          onPrimary: Colors.white,
          surface: Colors.white,
          onSurface: Colors.black,
          brightness: Brightness.light,
          background: Colors.white,
          onBackground: Colors.black,
          onSecondary: Colors.white,
          secondary: Colors.cyan, 
          error: Colors.red,
          onError: Colors.white,
        ),
        dividerColor: Colors.grey,
        textTheme: const TextTheme(
          titleLarge: TextStyle(
            fontWeight: FontWeight.bold,
          ),
          titleSmall: TextStyle(
            fontWeight: FontWeight.bold,
            fontSize: 12
          ),
        )
      ),