[Theme Horse Support Team]

Hi miri,
Which div background section you are talking about? If you are talking about the Customize -> Colors -> Background Color then this background color will only takes effect when you site is narrow layout.

You can set your site to narrow layout. Go to Appearance -> Customize -> Ambition theme options -> Site layout and select Narrow Layout. By default it is wide layout and Click on Save & Publish button at the top of the options panel.

Thank you!