This is just a personal view.
I think themes are great, but probably they lend themselves more to those who need to repeat a slide show many times. In that case the theme used is more likely yo be made by the author and therefore its known what slide styles are contained within the Theme.
Probably, the way to see what styles a theme contains is to quickly apply that theme to the number of images the theme states. That should take just a few seconds to set up and what better way to see if the theme meets your needs, better than a full screen preview