You could create a section in your css and name it “Custom” and keep all of your edits there if you want. You could also create a custom css file (separate) and link to it from the style.css file. This makes it easier to manage all of your edits in case of an update.
You could do that, or you could just edit the style.css file directly if you want, many people do. Either way is fine, it just depends how you want to manage updates to your css (if there are any).