I would like to suggest an easier way to mass delete unused themes. The option to delete themes within WP is great, it saves time instead of having to go through the FTP programs and guess at which themes need to be deleted. HOWEVER, it seems that you can only delete one individual theme at a time within the WP Dashboard area. It would be nice to be able to check a box and mass delete themes. Would save a lot of time and maybe encourage more theme installations.