Hi Tiago Silva,
I'm attaching an example project. It has the Office_Blue theme with modified window background. You can find the changes in xaml marked with "CHANGED" comment. I have no idea why it does not work for you since all I did is to change the xaml, set the theme and reference the theme project. I hope the attached solution will help you.
Thanks for your interest in our controls.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.