- The file templates you can find here: app/code/Magento/Checkout/view/frontend/templates/success.phtml In order to customize that you have to copy the file from app/design/frontend/Magento/luma/Magento_Checkout/templates/success.phtml
- Before designing your success page you can stumble across a problem. We mean when you click "Refresh" button on a success page, you will be redirected to the empty cart. Thus we recommend you find the file app/code/Magento/Checkout/Controller/Onepage/Success.php and comment this line in public function successAction.
Line is $session->clearQuote();
- Right now it would be better to check for compatibility with different browsers.