How apps like Hacker News beta have use the Ice Cream Sandwich 4.0 UI on their app and at time work without any problems on 2.3 Gingerbread and 2.2 Froyo? I want to create apps that should work on 4.0 and have backward compatibility like that, but how to do it?
Many apps also use ActionBar Sherlock to support the ActionBar UI. It's built on top of the Support Package that bobygerm mentioned.
You have to use the Support Package and its support libraries (v4, V7, v13) - Android Developer.
Some APIs are not yet included in this support package, it may not cover everything that you use.
You need to use the support library or only the visual UI, the resources available on Android ICS.
If you need only the resources, you can download the ICS sdk and copy the resources on
android-sdk-mac_86/platforms/android-15/data/res.
You can check this on this app:
来源：https://stackoverflow.com/questions/9886523/how-to-create-a-android-app-with-ics-ui-that-works-with-gingerbread-and-froyo