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.