Walkthrough: Dynamically Skinning your Windows 8 App
One powerful feature of XAML is the ability to make any control look however the developer or designer wants without having to change the control itself. Similar to, but far more powerful than CSS, skinning a control in XAML lets you change how it looks, change the type of control, and even inject behavior.
You can change its behavior, customize it’s API, and even add properties as needed. Taken to the extreme, it can actually be a little confusing. But used appropriately, it’s a powerful, time-saving feature.