If you do this, you'll loose all your apps which are installed in the phone. If you want to delete the skins, you need to delete C:\System\Skins
C/System/Skins/the theme you want to delete(it will be a number so open that file and the name of the theme will be in there),If they are applied to phone memory.
E/System/Skins/the theme you want to delete(it will be a number so open that file and the name of the theme will be in there),If they are applied to the mmc.
Dont delete the whole folder or every theme will be deleted!!!!!