Actually, if you create a new project based on the JetPack theme, you will see that there is a TiledBackground user control that performs a repeating effect for you.
However, the default ResourceUri dependency property of this user control is, in my opinion, binding to the wrong image file, and therefore you only see a light background color.
To have the same background as in the demo, you just need to replace the backgroundtexture.png image in the Images folder with the image file that Joe provided. Then in MainPage.xaml you need to make sure that SourceUri is attached to this image.
For example, I replaced the Joe image and still named it backgroundtexture.png and my project name is JetPack.
<controls:TiledBackground SourceUri="/JetPack;component/Images/backgroundtexture.png" />
Then you should see a dark background as expected. :)
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