How do you delete themes that dont show up in the installer?

If you've installed them on the card then connect the phone to the pc using data transfer mode. Browse the card and go to the 'private' folder. In there you'll find several directories, and within some of those directories you'll find sub-directories named 'import'. Browse within those deirectories and you'll find your themes in their own sub-directories in one of them (each theme has its' own directory). I deleted the directory the theme I didn't want was in and when I disconnected the phone it wasn't there anymore.

If you've installed the theme on the phone itself then I've got no idea.