Just to be thorough on my testing, I tried setting the theme on one of my WP7 to light. Surprisingly, it turned out that one of the grids was not as wide as I thought and two white bands appeared on either side. Examining the XAML should no reason for the bands. With the dark theme, black next to black does not show.
Luckily Blend is a bit more verbose than Visual Studio and it was easy to track down the grid that was not as wide as it should have been. One of the grids had a width of 471. Checking back in Visual Studio showed no explanantion for the gap. The 471 was not a fixed width because in landscape mode, the width appeared to be around 800.
The temporary fix was to set the width at 480 and then add code to the orientation routine to change the width from 480 to 800 depending on the orientation.
John Marshall… Visio MVP Visio.MVPs.org