Themes Admin

I have several themes installed. How do I uninstall ones I am not using. They still show up on the list of themes. One is broken because I updated OC over it after I discovered ti overwrote core files.

I also have two themes that I only seem to be able to use one of them. The list shows the "Default Store Theme" and then "Retroactives Store Theme" . If I enable a store in the "Default Store Theme" and set the folder to the theme folder, it works. If I enable it in the "Retroactives Store Theme" and set the folder, the site says "Error: A theme has not been assigned to this store!"

I have multiple stores being setup and would like to have a theme for each store. What is the purpose of the additional themes if they cannot be used?