Theming is a huge part of Android and on Android 4.0 (Ice Cream Sandwich) Holo is the main theme.
Holo & Holo Light Theme
Google have decided that to ensure consistency of applications across versions, Holo will become a mandatory addition for anyone using Android 4.0.
"Before Android 4.0 the variance in system themes from device to device could make it difficult to design an app with a single predictable look and feel. We set out to improve this situation for the developer community in Ice Cream Sandwich and beyond"
Don’t fret though as this won’t stop theming or OEM’s using their own style on a handset or tablet but it will stop apps looking different on one version of Android to another, Essentially what is being said is that if the device carries the Android Market then it will have the Holo theme.
Via: Pocket Now
Source: Android Developers Blog