All the skins we ship under /Data/skins have what is needed so you can compare to those.
Every skin we ship has this at the top of the theme.skin file to declare the controls.
<%@ Register Namespace="mojoPortal.Web.UI" Assembly="mojoPortal.Web" TagPrefix="portal" %>
this was not a new change it has been there a long time. the <portal:OuterWrapper control is a new control but it should be recognized from the pre-existing declaration as shown above.
Hope that helps,