Find your theme item (/sitecore/media library/Project/Tenant/Site/Site Theme) and check following check boxes:

Find your theme item (/sitecore/media library/Project/Tenant/Site/Site Theme) and check following check boxes:

This will add following css classes to the page body element:

• header-wide
• content-wide
• footer-wide

so your footer, header, content placeholders will have full width.