public static ApplicationTheme ApplicationTheme {get; set;}

{
  //Gets the application's theme
  var theme = FrameworkApplication.ApplicationTheme;
  //ApplicationTheme enumeration
  if (FrameworkApplication.ApplicationTheme == ApplicationTheme.Dark)
  {
    //Dark theme
  }

  if (FrameworkApplication.ApplicationTheme == ApplicationTheme.HighContrast)
  {
    //High Contrast
  }
  if (FrameworkApplication.ApplicationTheme == ApplicationTheme.Default)
  {
    //Light/Default theme
  }
}