No, there's not an API for changing it. System-provided UI like the file picker, message dialogs, credential picker, and so forth are styled according to the user's preferences. I believe this reinforces for users the idea that such UI behaves in the same
way across all apps, and that they don't have to look for subtle variations.
The one exception in the pickers, at least, is that if you choose an app like the Camera through the picker, you do see the styling from that app.
Kraig
Author,
Programming Windows 8 Apps with HTML, CSS, and JavaScript,
a free ebook from Microsoft Press
