Broaden Your Horizons with More Width Control
Decide how much space your content commands on a page. All default theme layouts in Lightning theme templates now include the option to set the maximum page width. Let your pages boldly span to the edges of the browser window, or set specific pixel size boundaries to control the width of the content across the page.
How: In Community Builder, select. Select Set max page width and use the slider to set the width. If you deselectSet max page width, the page width fills the page. This setting doesn’t affect the header, hero or footer, custom overrides, and existing theme layouts.