Google has made it clear that the "Holo" theme is mandatory for all Ice Cream Sandwich devices - to be specific, it is absolutely necessary for those looking for access to a very wide range of apps from Google, which includes Android Market.
But this move is nothing more than an effort to maintain the "consistency in the user interface." This will help developers create a "standard theme" no matter what the device is or who the manufacturer.
This does not mean developers are bound by any regulations - there are provisions in place for customising manufacturer and developer products. Manufacturers can still develop their own themes for Android powered devices. And App developers can choose from Holo or any other theme offered by other developers and manufacturers, reports Tech 2.
Google is calling the present form of Holo an "unmodified Holo theme" which looks somewhat like the Ice Cream Sandwich.
The essential theme makes the User Interface (UI) widgets and design elements that developers can use to develop crafts. In simple words, Google is putting a condition to provide further scope and opportunity to manufacturers and developers.