How to theme checkout completion page in Drupal Commerce
A quick google search yielded this result:  

Templating the Commerce Order Completion Pane  


  Turns out that's a fairly simple task thanks to
  hook_commerce_checkout_pane_info_alter. The function passes the
  variable $checkout_panes by reference which contains an array of
  checkout pane arrays. This function is going to allow us to redefine
  the function that the checkout completion pane is looking to for
  output.