There's no real documentation on this yet. I wrote a wiki page today that gives an overview. Many things have changed for themes, so many that to actually edit an old stylesheet to make it compatible seems like much more work than to start with a Tiki 3 stylesheet and modify it to get the look of your pre-Tiki3 site.
Anyway, please see Updating old themes for Tiki 3 for an overview. I'll add more details soon.