the UE behavior is on the confirm page and the pp_confirm page.
both pages use the wa_receipt.php page as the template for the email body.
edit the design on that page to change the look of the email.
If you are weary of making changes to that file, back it up fIrst.
be aware, that email clients do not read linked style sheets, so any css for the email is not done using classes like you would do on a web page, but through inline style, for example:
also, any images or links in the email body must use a fully qualified URL.