I once saw a tutorial where all u have to do is paste a short code into the head.php file & then another short code at the bottom of style.css
You shouldn’t be following any advice that tells you to modify the theme files directly. If the theme gets updated because of feature enhancements, bug fixes, or security patches, your changes will become lost. That’s why I recommended creating a child theme. If you’re adding shortcode, then that must mean you have to install a plugin? Although I don’t know of any shortcode that will work inside a CSS file. You’re better off making the change that I suggested above. That way, only one file needs to be modified, and it’s a very simple change.