Welcome to our community, Patty
Try selecting all the slides in either Storyboard view or
your Film Strip. Right-click the selection and examine the Slide
Properties. Once there you should see an option allowing you to
change the Background Color.
Keep in mind, however, that if your project was recorded,
each of the backgrounds will consist of screen captures. Thus any
change to color won't really affect anything.
If you are creating a project from scratch, you might look at
Edit > Preferences... > Project node > Settings node.
There you will also find a global option for setting the background
color.
Cheers... Rick