I do my editing on my computer so I have two folders – one for a backup and the other folder labeled “working theme”, that’s where I do all my edits.
Before I do an edit I sometimes make a backup of that file that I’m working on. For example, the style.css is the orignal file. To preserve that file before I work on it I’ll make a copy of it and name it style.css.saveme [or today’s date]. This way if I screw up the style.css file that I’m working on, I have a working copy to fall back to. I use an FTP client to upload my changes and refresh the site to see the changes.
You can make a copy in same folder, just make sure its labeled something like I mention above.
I’m not familiar with prose or how that works so can’t help you there.
Question: When you say “I was using Prose, which directs you to edit css via WP Admin/Genesis/Custom Code” is that the same way as going into your WP dashboard > Appearance > Editor and selecting style.css?
By the way, it’s okay to be a newbie. We were all there and at times, I still feel like a newbie. So good on ya that you’re learning and doing. Keep it up.
Hope this helps you.