All your style will need to be updated. That's because 1.2.x is a responsive design which also acts as a mobile friendly design. XenForo had the good sense to incorporate it all into 1 single layout.
As for the add-ons, it's not a bad as most people thought it was going to be. Having come from IPB where every update meant having to upgrade just about every add-on I was using.... I was very worried about upgrading to XenForo's 1.2 because I had that mindset in place.
When I did the upgrade I had over 100+ add-ons installed. Out of all of them, only 5 were truly problematic, but not so problematic that I couldn't easily remove, disable, or update them.
XenForo 1.2.x upgrade overall was painless and easy to manage.