1. On the selected page, click the pencil icon to bring up the style panel in the sidebar.
2. One method is under the "Background" tab, there is a "Darken background" toggle option. Click to turn on a default black subtle overlay.
3. A second method is to turn on the "Gradient background" toggle underneath. You can set custom colour values instead of the default black opacity if preferred.
Feel free to keep playing around to see what looks best. Here is an example before and after: