Description of problem:
Gnome screen shield has a blue color and it can't be changed to neither other color or image.

Version-Release number of selected component (if applicable):
Gnome version 3.28.2

How reproducible:
Always

Steps to Reproduce:
1. Open Settings->Background->Screen shield.
2. Choose another color or image.

Actual results:
The screen shield is the same.

Expected results:
The shield has a color that I set.

Additional info:
On command line, the command
$ dconf read /org/gnome/desktop/screensaver/picture-uri
returns
''
However, when I try to set an image to the key:
$ dconf write /org/gnome/desktop/background/picture-uri "'file:///usr/share/backgrounds/8-dark.xml'"
it returns
error: The operation attempted to modify one or more non-writable keys