Path of css depends on the template you use but generally in Gantry 4.x we use LESS files and not CSS files. If you need to make some changes, override css definitions in custom css files
When we release upgrades template files will change and you will have to reapply all your changes again (that's if you can remember where they all are). If you keep your changes in a separate Custom CSS file you'll have no issues upgrading.
Path of css depends on the template you use but generally in Gantry 4.x we use LESS files and not CSS files. If you need to make some changes, override css definitions in custom css files