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In an effort to create a consistent experience for its users, Google is requiring manufacturers who ship their devices with Ice Cream Sandwich (4.0) to include Holo, its default theme intended for a "pure" Android experience. Google will still allow manufacturers to modify android with their own custom interfaces, widgets and skins, but an unmodified Holo theme must be present.
If a future of impersonal, monolithic Android phones and tablets frightens you, don't worry just yet. The company claims, "We have no desire to restrict manufacturers from building their own themed experience across their devices." Google also mentions it is actually easier to theme the platform than ever before.
Holo first appeared in Honeycomb (3.0) but the requirement to include it only extends to Ice Cream Sandwich. Devices which are required to comply with this guideline must also utilize the Android Market. Although the overwhelming majority do, not all Android devices have access to Google's official market place. This would excuse devices like the Kindle Fire which uses Amazon's own curated app store, ignoring fact it runs an older version of Android to begin with. By the way, despite this limitation on the Kindle Fire, there is a way around it.
In Android 4.0, Holo is different. We've made the inclusion of the unmodified Holo theme family a compatibility requirement for devices running Android 4.0 and forward. If the device has Android Market it will have the Holo themes as they were originally designed.
As a result of the changes found in Ice Cream Sandwich, developers may now explicitly call upon the plain old vanilla theme for their apps. This may actually help relieve the tension created by trying to give apps a consistent look and feel amidst a plethora of custom UIs found on millions of Android devices.
According to Google, improvements made to the theming system should also allow manufacturers to upgrade their devices to major platform versions more quickly and with less effort. Presumably, this is because companies will have less to mull over in terms of maintaining their custom UIs across versions. Perhaps this will encourage companies to adopt and update future versions of Android more quickly.