Premiere Elements 13 on what computer operating system?
If you are referring to "themes", you are in the Elements Organizer 13 Create/Slideshow and not in Premiere Elements 13 Editor.
The Elements Organizer 13 Slideshow Builder offers a lot of bells and whistles but does not have all the editing opportunities of the Elements Organizer of pre-13/13.1 versions.
Elements Organizer 13 slideshow slide duration is according to theme preset. And, export from the Elements Organizer is only as 720p or 1080p AVCHD.mp4 file saved to the hard drive or Facebook. That save can be imported into a Premiere Elements project but there is no more "break down Elements Organizer Slideshow" opportunities for that slideshow in the Premiere Elements 13 Editor as had in output "Edit with Premiere Elements Editor" in pre-13/13.1 Elements Organizer slideshows.
Have you considered creating and exporting your slideshow solely in Premiere Elements 13 Editor?
Please consider. Any questions or need clarification, please do not hesitate to ask.