This will never happen because:

1. Companies will always want to brand their apps with their particular UI styles.

2. In order to prevent the above, the OS would have to deliberately NOT expose the ability for apps to control their own pixels.

Doing 2 means you are making it impossible to support many application types (photo editors, games, etc.).

NOT doing 2 means that app companies will eventually use the same APIs that the photo editor and game applications use.