To set a full-screen background with the page content scrolling on top, there's a few things you need to do:
Set the page layout to Stretch to the viewport height and Disable scroll under the Style tab
Add a Container component, give it a background image and set its height to the viewport height system variable with the formula
(Due to a known issue, the view canvas won't show the container height correctly, but you can circumvent this by setting an explicit minimum height value)
Inside the container, add a Scroll View, and set its height to the same formula
Add your scrollable content inside the scroll view
Check the tutorial video below for the full walkthrough!