The CSS files for Magento 2 are generated from several less
files. When you run your webshop in developer mode (on your local machine), and then running the grunt commands (I usually run grunt exec:<theme> && grunt less:<theme> && grunt watch:<theme>
), your browser will show the less
file that contains the styling of the selected element.
If this isn't working for you, it could also help by finding a specific piece of styling, like the class, can be tracked back to one or more less
files in your installation. This will most likely make it possible to track back where the original styling is declared. Be aware, this can also be in the parent theme (could be the blank theme).
php bin/magento setup:static-content:deploy
command ?