This is actually a bug/limitation in dnn when you redirect to another user control 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

This is the technique I have been working on but it’s kind of a big change and will require a lot of testing before its release

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 I try to provide a fix for this in the next release.

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At your service,
Dave Smith

That would be a much bigger undertaking then the method I am working on now. Additionally, I think people appreciate the user friendliness of being able to drop one module on a page and the entire catalog, cart and checkout is encasulated in one module, very easy to understand and setup your ecommerce site that way.

Appreciate your feedback.

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At your service,
Dave Smith

The only real downside I see is that the user cannot see the MiniCart when they are viewing the product details. Is there any way around this?