// Determine the visibility of the dark background.
Visibility darkBackgroundVisibility =
    (Visibility)Application.Current.Resources["PhoneDarkThemeVisibility"];

// Write the theme background value.
if (darkBackgroundVisibility == Visibility.Visible)
{
    textBlock1.Text = "background = dark";
}
else
{
    textBlock1.Text = "background = light";
}