We support full customization of application theme. Simply set themeBuilder in your main:

void main() async {
  await FlutterUI.start(
    FlutterUI(
      ...,
      themeBuilder: (themeData, seedColor, brightness, useFixedPrimary) {
        ThemeData theme = themeData.copyWith(
          appBarTheme: AppBarTheme(backgroundColor: themeData.colorScheme.surface),
          elevatedButtonTheme: ElevatedButtonThemeData(
            style: ElevatedButton.styleFrom(
              shape: const RoundedRectangleBorder(borderRadius: BorderRadius.all(Radius.circular(25)))
            )
          )
        );
 
        return theme;
      },
    ),
  );
}

In this example, we set buttons rounded.