Associated blog post: http://jason-mitchell.com/windows-phone/theme-aware-panorama-control-for-windows-phone/

SamplePage.xaml

<local:ThemeAwarePanorama Title="my app"
                          LightBackground="/MyApp;component/Content/Images/Light/Background.jpg"
                          DarkBackground="/MyApp;component/Content/Images/Dark/Background.jpg">
    <!-- more xaml -->
</local:ThemeAwarePanorama>

ThemeAwarePanorama.cs

public class ThemeAwarePanorama : Panorama
{
    public static readonly DependencyProperty LightBackgroundProperty = DependencyProperty.Register("LightBackground", typeof (string), typeof (ThemeAwarePanorama), new PropertyMetadata(default(string)));
    public static readonly DependencyProperty DarkBackgroundProperty = DependencyProperty.Register("DarkBackground", typeof (string), typeof (ThemeAwarePanorama), new PropertyMetadata(default(string)));

    public ThemeAwarePanorama()
    {
        Loaded += (sender, args) =>
        {
            if (!string.IsNullOrEmpty(LightBackground) && !string.IsNullOrEmpty(DarkBackground))
            {
                string url = UIHelper.IsLightThemeEnabled() ? LightBackground : DarkBackground;
                Background = new ImageBrush {ImageSource = new BitmapImage(new Uri(url, UriKind.Relative))};
            }
            else
            {
                throw new InvalidOperationException("Light and dark background must be provided");
            }
        };
    }

    public string LightBackground
    {
        get { return (string) GetValue(LightBackgroundProperty); }
        set { SetValue(LightBackgroundProperty, value); }
    }

    public string DarkBackground
    {
        get { return (string)GetValue(DarkBackgroundProperty); }
        set { SetValue(DarkBackgroundProperty, value); }
    }
}

UIHelper.cs

public static class UIHelper
{
    public static bool IsLightThemeEnabled()
    {
        return (Visibility) Application.Current.Resources["PhoneLightThemeVisibility"] == Visibility.Visible;
    }
}