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In the AppUI's second-phase constructor call the BaseConstructL() method and passing EAknEnableSkin as its argument:
// Enables skins in all the optionally
// skin-providing controls in this app
BaseConstructL( EAknEnableSkin );
This will ensure that all optionally skin-providing controls will have skins enabled. If you do not want to enable skins, then simply leave out the EAknEnableSkin argument.