There are 2 different background theme properties for pictures. There is one located at Themes → General → Background → Background picture. This one is used as background for the application when the List bar menu is active. The other one is located at Themes → Navigation → Tiles → Tile menu → Background picture. This one is used as background for the application when the Tile menu is active.
It would be nice to inherit the Tile Background picture from the General Background picture when nothing is specified.
That simplifies the theme setup.