For any kind of "theme" the window listing them is not loading the previews, either as list or grid view. You need to hit details in order to see them.

BUT you only get to see one with a big thumbnail. If there is more than one available and you click on any of the others the big thumbnail will disappear and the one you clicked will never load.

Also ticking "installed" bullet on the right will not show previously installed themes nor the previews for them.