Custom theme from existing theme

Hi, I'm new to Captivate and trying to create a custom theme to be used in all of our company's projects. 
I've watched some videos of how to transform an existing theme into a new one, but those were old so Captivate looks different now. When I tried to change the masterslides it looked different when I exited Masterslide. I’ve also read the user guide but couldn’t find the explanation I was looking for.

I'd very appreciate if someone is willing to walk me through the proccess basics on a meeting.