Actually, the v6 "theme" is far more customizable than v5 - you use the Glade and GK+ WYSIWYG editors to work with it, you can change pretty much anything.
True, for "simple" use only the background is exposed in ePO, but as I say, unpack the zip and you'll see you can edit pretty much anything, either by hand by massaging the various files, or using Glade.
Yes, there is a readme.html included in the documents.zip. It has known and resolved issues and so on. I still have trouble with my own user (password not accepted), but I start from scratch at the client (VM) now.
Just wanted to provide an update on this topic. A new 6.1 beta has started this week. It is a private beta, so please message me or work with your account team to get access to the beta code. The plan is for the 6.1 beta to go from Jan 17th to Feb 18th.