有两种方式可以获取到

1. 不使用Context

final brightness = SchedulerBinding.instance.platformDispatcher.platformBrightness;
bool isDarkMode = brightness == Brightness.dark;

注意！如果是在程序入口使用，需要加上下面语句

WidgetsFlutterBinding.ensureInitialized(); 

2. 使用Context

final brightness = MediaQuery.of(context).platformBrightness;
bool isDarkMode = brightness == Brightness.dark;

OR

final brightness = View.of(context).platformDispatcher.platformBrightness;
bool isDarkMode = brightness == Brightness.dark;