Those images aren't a part of the theme, they are a part of the specific demo. If you add a Tabs component to the center of the border layout UI you will get thee tabs where the "icon" property will allow you to customize the unselected icon and you can customize the selected icon as well. You can see the full source/resource file of the social boo demo in the docs and demos zip in the download section.
Generally customizing commands is a bit complex because of platform specific constraints, when running on an Actual Android 4 device we use the native ActionBar by default to provide the experience Android users are used to. This is hard for us to simulate so you will only see this properly in the device build. I suggest not relying on commands, it seems that what you are trying to achieve is easily doable with tabs and its possible that's why you had difficulties in your other question.