At the end of the day, this is going to be done in a similar way to how you skinned previous ADF applications. (If you have never done this before, you might want to watch the videos in these two blog entries).
But since the skinning design time is not completely there for Alta in JDeveloper 12.1.3 there are a couple of tricks. Specifically when you create the new skin, you'll need to change the trinidad-skins.xml file to indicate it is extending the alta-v1 and not skyros-v1 - <extends>alta-v1.desktop</extends>
Then the rest of your tasks would be basically the same (although you won't see the overview tab in your skin editor).
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