Yes, everything is possible with skins.
TabBar is part of TabbedVieNavigator and also TabBar is inherited from ButtonBar. You should to create skin for ButtonBar and set it for your TabbedViewNavigator #tabBar like this:
<fx:Style>
@namespace s "library://ns.adobe.com/flex/spark";
s|TabbedViewNavigator #tabBar
{
skinClass: ClassReference("views.skins.TabbedViewNavigatorTabBarSkin");
}
</fx:Style>
where TabbedViewNavigatorTabBarSkin is Skin of ButtonBar host component, please look at the screen below:

Then you just creating you own skins for ButtonBar buttons and set own BG color and icon and other things, which you want. Hope this help you.