Captivate is a Theme based application. A Theme has a lot of components: theme fonts, theme colors, object styles and master slides to mention some. Each slide in a Captivate project is based on a master slide, but most Themes have a Blank master slide which takes on only the Background color/gradient/image defined in the Main master slide. Master slides can be compared with what you find in PPT.

You add a slide by using either the big button Slides, and you choose one of the master slides to be used, or by using the Insert menu, New Slide From where you can also choose a master slide as foundation for the slide.

I don’t know if this is answering your question, or do you need more information? Since version 11.5 it is also possible to have multiple themes in a project but I wouldn’t recommend that to start with.

Have a look at:

https://blog.lilybiri.com/whats-in-a-theme-a-template