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
}