This extension is the default product layout for Shadows. The CSS is part of /themes/%STORE_ID%/THEME_NAME/extensions.css. If you want to make styling changes, it is recommended you add them to your /themes/%STORE_ID%/THEME_NAME/theme.css file.
If you are using Shadows DE, you can modify the CSS or JavaScript in build/extensions/product-layout/ and update as part of your build process. This extension includes the ajax-add-to-cart.js functionality as well as the styles for the product photo gallery.