Hi there,
most of the GP Sites use the Block Element to add custom design components.
When you view a page check the Admin Bar > Elements menu to see if there are any active, as well as the shortcut link to find them, which is in Appearance > Elements.
In the case of Stream it has one element titled Global Footer.
Edit that, and select the top most Container Block, and in the sidebar Backgrounds change the Image URL
Note that image Opacity is also set to 0.1.
And there is a Background Color set in the Colors panel.