a) In general, I like your design, though I'm not a big fan of the flicker when you use Javascript to change the font. I'm using Chromium on Ubuntu though, so this may not show up on Windows/OS X.
b) Check out this scenario. Enter your live demo site and go to the settings page. Attempt to change a value on that page and then click 'save'. When I save the settings, I end up on an unstyled page that says, "{"success": "Store Settings Successfully Saved"}". I suspect that would scare customers away!
c) The 'add a product' page needs some work. I found the whole process quite difficult and found myself constantly asking myself what information I really need. This is problematic because I suspect I have more eCommerce experience than your target customer. The first thing I'd do is move the 'add photos' function higher. Second, I'd consider walking people through the process (ie - through a series of smaller forms rather than one long form)...