How to make the site full screen width?

Something that's bothering is I can't make my interfaces take up the whole width of the screen. I have se the setting on the site to be "Full" but the interfaces still show empty spaces on left and right sides. One of the screenshots on this reference appears to suggest that you can make an interface take up the whole width. https://docs.appian.com/suite/help/21.1/ux_page_width.html

The only thing that can take up the whole width is a billboard layout inside a header content layout, but nothing else. Any suggestions?

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