If You want to update from Quick.Cart v4.0. Check changes using Total Commander. Select file in Quick.Cart v4.0 (first column) and the same file in Quick.Cart v4.1 (second column) and click in menu "Files" > "Compare by Content". You will find changes made by us.
Nice job, but I don't like your CSS. I looked at v3. That's perfect. Easy to change, very flexible. In v4... that's something like ****. Conaitner main container main container main.. that's terrible. It's very hard to understand for new users who wanna change the skin. For experienced users it's better but not better than in the earlier version. I've never met anything so ***** in styles like in your current version. And default skin is not attractive. I like your functionality, but I don't like your CSS. I tried to make a template on v3 and v4. v4 took 200% more time to implement that. I'm shocked.
I had to use the <div class="clear"></div> a lot in these containers and mains, because in Opera the divs were collapsed in height (even though they had some content inside ) unless the clearing div was used in them properly.
I have a suspicion that the devs used the different containers for solving of the IE box model problem. It's not ideal due to the over-complicated structure, but it works. Perhaps Treewood could share some light on this, I might be wrong.