The above answers are working except for the Ok/Cancel buttons. Just adding this in case somebody needs help on customizing it. It is a combination of colorScheme and buttonTheme.
showTimePicker(
context: context,
initialTime: TimeOfDay(hour: hour, minute: minute),
builder: (BuildContext context, Widget child) {
return Theme(
data: ThemeData.light().copyWith(
primaryColor: const Color(0xFF8CE7F1),
accentColor: const Color(0xFF8CE7F1),
colorScheme: ColorScheme.light(primary: const Color(0xFF8CE7F1)),
buttonTheme: ButtonThemeData(
textTheme: ButtonTextTheme.primary
),
),
child: child,
);
},
);