The new page editor has a fantastic background option, the only thing it's missing is the option to add a background overlay.
To fix this issue, Atomic comes with a module called Section Settings. You can drag-and-drop it to any location within a section.
Once added, open the module from the Contents tab:
Check the "Overlay" boolean to enable the option, then it will show a "Color" field under Overlay settings where you can choose a custom color and opacity.