If you are using Fedora, the easiest option is to install the desktop-backgrounds-gnome
.package (I’m assuming you aren’t using Workstation or Silverblue, where the package is included out-of-the-box).
That package installs a /usr/share/glib-2.0/schemas/10_org.gnome.desktop.background.fedora.gschema.override
file with the following contents:
[org.gnome.desktop.background]
picture-uri='file:///usr/share/backgrounds/f36/default/f36-01-day.png'
picture-uri-dark='file:///usr/share/backgrounds/f36/default/f36-02-night.png'
You can also create such an override yourself. In that case you’ll have to run
# dconf run
after saving the file.
Also see the system administration guide for documentation.