Something went wrong today while WordPress was updating one of the themes I have installed. The theme seemed to be installed but the control panel reported that the stylesheet was missing and when I tried to reinstall the update it said the theme was already up-to-date. I couldn’t even uninstall it! I searched the web for a solution and the first thing I clicked on was this ancient post. It links on to some instructions, that are probably wise to follow, if you have customized your theme or if it’s in use but for me neither was the case, so I simply deleted the theme folder (which was empty by the way). After that I could reinstall the theme.
I’m not sure how common it is to have unused themes installed, just for reference, but I have a few that I like and consider using in the future, and this way seems to be the easiest way to keep track of them. It’s not that they take up a lot of space.