Color error help

[llimental]

As of February 14, 2022.
If you have a problem with changing the color of the hex code in the latest version of the Flutter, please refer to this code.
I'm confused to solve everything in ThemeData, and I just need to color the ScaffoldBackgroundColor in ThemeData, and Title and accent(now not used) respectively.

[nonsocchi]

instead of specifically setting the FAB to purple, use

class BMICalculator extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        colorScheme: ColorScheme.fromSwatch(accentColor: Colors.purple),
        scaffoldBackgroundColor: const Color(0xFF0A0E21),
      ),
      home: InputPage(),
    );
  }