Where is the "billing information" page template d

[dnrbookstore]

I've created a new theme based on the modern theme. So far i've been able to find all the areas where the template a page uses is defined with the exception of the 'billing information' page. When I click 'checkout' and go to the page where the billing information is to be input it no longer has the same look as my adjusted theme pages. I originally thought this was part of the modern_Cart.sst, but the cart page looks to be using my new template. So I assume its using an original template file from the modern theme. I can't for the life of me find where the template that the 'billing information' page uses is defined so I can change it. Any help?

[BFChris]

All the steps in the cart are contained in the single cart template file. There are separate html pages for each separated by [-- DEFINE xx --] tags.

The Billing Information page ("Step 2") is defined inside the
[-- DEFINE Shipping --] tag.