I have added a bounty on StackOverflow to my issue:
http://stackoverflow.com/questions/36093271/how-can-i-use-blend-or-vs-designer-to-edit-default-style-template-for-wpf-proper?noredirect=1#comment59911298_36093271
http://stackoverflow.com/questions/36093271/how-can-i-use-blend-or-vs-designer-to-edit-default-style-template-for-wpf-proper?noredirect=1#comment59911298_36093271