@Chip Bennett - "If not, just close the Customizer without saving changes, and then delete the Theme"

This rather defeats the object of the game. It means that I need to download a theme and then upload it to my server before I can see whether it is one that I like, one that fits in with my project. How many thousand themes are there on WordPress.org? If I just cut it down to simple things like 2 column, blue... still a lot.