This is actually a bug/limitation in dnn when you redirect to another user control within a module dnn applies the admin skin instead of the portal skin.
http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/107/threadid/46342/scope/posts/Default.aspx
In Smith Cart v3.85 release the product details page has been split out as a separate module which will apply your portal skin to the product details page so if you install v3.85 the product listing and details pages will use your portal skin but the cart page will still use the admin skin.
Long story short you will need to set your portal and admin skin to the same skin temporarily if you want the same skin on the cart page.
-Scott