How do I theme checkout pages? how do I theme the cart and checkout pages in Drupal commerce?  I have set content, account, billing, and payment parts. Is there any --.tpl.php file to create?
For theming cart page add page--cart.tpl.php in your theme's templates directory.

For theming checkout page add page--checkout.tpl.php in your theme's templates directory. If you want to theme only the checkout review page add page--checkout--review.tpl.php in templates directory.

After adding tpl files do not forget to clear cache.

If you want to know about more hooks do a dpm($variables) inside YOURTHEMENAME_preprocess_page function and look into the theme_hook_suggestions array.

For details read this comment http://drupal.org/node/1142800#comment-5365466