The product details and category browse page PHP files are located here:
Category browse page: ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/category/default.php
(default.php, defaultlist.php, flexv2.php, or flexv2list what ever you are currently using, there are 4 different category browse page layouts and you are using only one of them depends on what ever you chose from VM Configuration)
Product Details page: ROOT/templates/<yourjoomlatemplate>/html/com_virtuemart/productdetails/default.php
(default.php or flexv2.php. There are 2 different product details page layouts and you are using only one of them depends on what ever you chose from VM Configuration)
Then you can stylize this SKU text with using the CSS and the .ProductKU class name. Something like;
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