droppyale, I'm sure nearly all the files recieved either some kind of changes, or tweaks to coorespond with any database changes. Meaning any changes you've made to your copy, upon update, will need to be done again. There is really no known set of diffs/change logs other than what mark keeps for us on the wiki which isn't exactly specific.
CSS changes, no, not really. If you're running a closed forum, and it's fairly small, you might be able to hold out (for a short while) and just wait for 9.3, but I'd still suggest patching to the latest stable release. 9.3 is looking promising enough that any major changes shouldn't impact anything you've handcrafted in terms of css / templates.