At the same time, everything could be reconfigured at will, but if you can't provider a consistent UX, that wouldn't help a whole lot with sales and customer binding.

How much you can, or can't modify as an end-user probably relates to the HIG and general usability and transferability as well.

In some ways, there have been cases where you cannot change the background of things, or at least not in an obvious way. I suppose it really depends on the specific user and their user-case to have that matter (or not matter at all).