Hello Paul,
The background will be controlled by your Color Palette in Customize -> Colors, but if you just want to change this specific background, you can do so using Custom CSS.
In order to select just the page title background, use custom CSS similar to this:
.entry-title a { background: gray !important; }
This example just uses the keyword gray, but you can substitute a hexadecimal color code such as #808080 or the shade that matches your color palette.
I hope this helps!