getThemeModeIndex property
      
      int
      get
      getThemeModeIndex
      
    
    
根据主题获取下标
Implementation
int get getThemeModeIndex {
  switch (this) {
    case ThemeMode.system:
      return 0;
    case ThemeMode.light:
      return 1;
    case ThemeMode.dark:
      return 2;
  }
}