Both Video Demo and Software simulations use the Blank master slide, whatever the Theme. They will also ‘fill’ the slides.

Like Ishan, I am not sure what you exactly want to know. If you want to add more objects to the slides of a software simulation, you could have a look at this blog post:

http://blog.lilybiri.com/embed-software-simulations-knockout-slide

However if you want to edit the recorded backgrounds of the software simulation, find them in the Library (use right-click menu) and use the popup menu ‘Edit with…’. Choose a graphics application, edit the background and when saving it will be changed in Captivate (roundtripping).

Do you mean you want to add a background to your Adobe Captivate slides after recording a video demo or a software simulation?