You’ll have to manually update your theme if you want to keep your css changes.
Since the Genesis Framework has made many updates and changes, that means the child theme will have to as well. You can manually do it all if you wanted to and it should work fine…..but there’s not really an easy way to explain other than just going through and manually doing it.
You could take a look here – http://appfinite.com/topic/legacy-1-3-for-genesis-2-0-html5-is-now-available/ but that’s just the basics to point you in the right direction.
I’d use something like diffchecker.com to compare the differences. The portfolio file had to be redeveloped to work with the latest version of Genesis.
I create awesome sites for awesome people! Contact me if interested – email@example.com