When you first start your presentaiton, you are prompted to choose a theme.  When you choose a theme, you're choosing the background for each slide in the presentation. 

Each slide in a presentation has a background. Some backgrounds are solid colors, and others are patterned or show parts of a larger image. Google Slides does a nice job of coordinating the background color with the color of the text and other objects on the slide. 

It is important not to choose a theme that includes backgrounds that are too bright or too distracting because they can make it hard for an audience to focus on the information in your presentation.

You will choose a theme soon - just click NEXT for now.