Anyone having promblems with their themes not loading or being unable to edit or save new themes in a-store....I am John:
you should delete your cokies on you computer and then try it again. Or change your computer to access to amazon.
May be I have the same error, when I change some color, the astore didn't update. But I found the way to make it work: Change to any theme except your custom theme, click to preview it. Then change back to your Custom theme and preview. Hope that work for some way!