An even more fiendish problem kept me from being able to add themes: a duplicate wp-content folder! It was on the same level as the wordpress folder! It even had a themes folder inside it, with the default and classic theme folders! I had tried a half-dozen themes, so when I dragged their folders into the *real* wp-content folder inside the wordpress folder, they all showed up! No screenshots for some of them, but there they were, and they worked when I tried them.
So, make sure you are using the right wp-content folder!
