Theme-aware XAML resources in a WP7 project - xaml

Question

I'm making a Windows Phone 7 application and I'm a bit confused with dark/light themes.
With a panorama, you very often set a background image. The issue is it's very hard to make a picture which is right for both dark and light themes. How are we supposed to proceed?
Is there a way to force a dark/light theme for a panorama? This will avoid making theme-specific panorama background pictures. Then how do I do? I found xaml files in C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v7.0\Design. If this is a right way to proceed, how can I import them for my panorama?
Or if there's no way (or if it's wrong) to force a dark/light theme: how to write conditional XAML to set correct resources? Now I have the following XAML (default.xaml) which is fine with the dark theme:
<ImageBrush x:Key="PageBackground" ImageSource="Resources/PageBackground.png" Stretch="None" />
<ImageBrush x:Key="PanoramaBackground" ImageSource="Resources/PanoramaBackground.png" Stretch="None" />
But when I use a light theme, black controls and black texts are hard to read with my dark background pictures. So I made different pictures that I can use this way:
<ImageBrush x:Key="PageBackground" ImageSource="Resources/PageBackgroundLight.png" Stretch="None" />
<ImageBrush x:Key="PanoramaBackground" ImageSource="Resources/PanoramaBackgroundLight.png" Stretch="None" />
Now my issue is to make XAML conditional to declare the right thing depending on the current theme.
I found no relevant way on the Internet. I would prefer not to use code or code-behind for that because I believe XAML is able to do this (I just don't know how).
EDIT: Code snippet to load a xaml file as ResourceDictionary
string xaml = null;
StreamResourceInfo xamlInfo = Application.GetResourceStream(new Uri("light.xaml", UriKind.Relative));
using (StreamReader sr = new StreamReader(xamlInfo.Stream))
xaml = sr.ReadToEnd();
dic = (ResourceDictionary)XamlReader.Load(xaml);
this.Resources.MergedDictionaries.Add(dic);

Answer 1

To force a dark or white theme you can indeed use the styles defined in the folder you pointed out. Copy and Paste the rules you need to your App.xaml (just PhoneForegroundColor, PhoneBackgroundColor and the related Brushes would be a good start).
It's probably better though to stay "theme-aware" and load a different image for light and dark themes. Here is an article explaining how to do this: http://blog.jayway.com/2010/12/16/theme-aware-panorama-background-in-windows-phone-7/

Answer 2

There is another possibility I've found: You can use the Coding4Fun Toolkit Converter according to these instructions. However, I'm unable to use correctly use them.
Another possibiliy is to use an OpacityMask. But this only works for black/white images :/
Yousef's solution looks interesting. but it takes too much time to load. The image will be changed round about 1s after the app started. I've tested this on a Nokia 820. I've moved the call for setting the DataContext in a Loaded Event, which was called much later. Now the call takes place in the constructor, so the image will be already set when the application displays it. However, it still adds more loading time :( Any suggestions on how to improve this?