5 Ways to Personalize E-Learning Courses - Background colours & Master Slides with Dave Anderson

You set up the background shapes on the slide. As per David's video, you'll save yourself a lot of effort by setting up the background shapes on the slide Master. 

You access the Slide Master from the VIEW ribbon: 

Here's the User Guide info about working with Masters: Slide Masters in Storyline 360 - E-Learning Heroes (articulate.com)