Full-width background images in Fixer

Hi there, Fixer appears to have a full width background image slider in the demo theme, which is why I chose it. However, I don't see how to do this in the customiser. Can you please help? I want to have a different background image in different sections, with images and text on top. Do I need to do this in the CSS, or is there another way?

In fact, from the customiser I really cannot tell at all how to change the background.
Thank you