This is already supported for both the Home and Health screens! Just navigate to the bottom of each screen and tap on "Edit Home" or "Edit Health".

This will open a pop-up that allows you to customize the background (i.e. the wallpaper), as well as the order in which each section appears.

And just for awareness: our product team is currently working on a major redesign to the Home screen that we hope to launch in January, that we think will be well received.