To add a CSS file to the popup window that appears when a shopper clicks on the print icon of a product page do the following:
Copy the file "components/com_virtuemart/views/productdetails/tmpl/default.php" to your template directory at folder: templates/my_Joomla_template/html/com_virtuemart/productdetails/ (if it's already there, then go to the next step)
Edit file templates/my_Joomla_template/html/com_virtuemart/productdetails/default.php and just after the line:
defined('_JEXEC') or die('Restricted access');
insert the following code:
$doc = JFactory::getDocument();
$css_path = JRoute::_(JURI::base()."templates/my_template/css/my_product_page.css");
$doc->addCustomTag('<link rel="stylesheet" type="text/css" href='.$css_path.' media="all">');
This way the file "templates/my_template/css/my_product_page.css" will always be inserted in the product page even if this page appears in the popup window of the print icon.
In this css file you can put a style like this for example:
body.contentpane .main-image img {
width:200px;
border:red solid 1px;
}