Since epubs can be pretty different, and what's needed or desirable for a styles configuration can be quite variable between documents, I wonder if we might be able to keep styles configurations pertaining to particular books, instead of having one styles setting, based on the last configuration when the viewer closed (thus any document will switch to this 'last' configuration when opened again). Right now I have a text document with a whole bunch of different styles settings that I copy and paste for particular books.
I suppose if this was done it would either save some data in the document that the viewer knows to read, or it could simply exist in the calibre library's memory of each book (in the way that it remembers the location you last read).