Holo Theme Is Now Mandatory For Android Devices
tripleevenfall writes in about the new theme changes in Android 4.0. From the article: "Starting with Android 4.0, support for the 'Holo' theme will be mandatory for phones and tablets that have the Android Market installed. Holo is the stock Android theme, known for its sharp angles, thin lines and blue hue. Third-party developers can now create apps and widgets using the default Android aesthetic, knowing that's how it'll look on every major Ice Cream Sandwich device that has the Android Market. "
This is not banning custom themes; instead it is merely giving developers a consistent theme that is guaranteed to be installed if they want a consistent look across all devices. There are even a few improvements to the style protocol to help developers deal with dark and light themes.
There's no wall here at all, just a foundation. You can still do whatever the heck you want with your device as a consumer, and this is just saying that manufacturer's shouldn't completely break the underlying UI structure, even if they want to supplant it with some theme of their own. "Do what you want with the field, just don't salt the Earth so nobody else can use it".