This extension is the default product list layout for Shadows used for sections like the category and search pages. 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 in build/extensions/product-lists/
and update as part of your build process.