How to change default desktop plasma settings of newly created activities?

Help
1

I would like to change the default desktop backgrounds of newly created activities. I would like to have all backgrounds black.

I succeeded with changing the following file. New activities have now desktops with black background on all three monitors.

/usr/share/plasma/shells/org.kde.plasma.desktop/contents/defaults

But it is now not possible to right click on the desktop to get the menu and because of this it is not possible to change the background on a created activity.

The content of the file looks like this now:

[Desktop]
Containment=org.kde.plasma.desktop
ToolBox=org.kde.desktoptoolbox
RuntimePlatform=Desktop

[Desktop][ContainmentActions]
RightButton;NoModifier=org.kde.contextmenu
MiddleButton;NoModifier=org.kde.paste
wheel:Vertical;NoModifier=org.kde.switchdesktop

[Panel]
Containment=org.kde.panel
ToolBox=org.kde.paneltoolbox
[Panel][ContainmentActions]
RightButton;NoModifier=org.kde.contextmenu

[Wallpaper]
Plugin=org.kde.color
FillMode=2

[Wallpaper/org.kde.color/General]
Color=0,0,0
2

The background changes to the standard image as soon as the kde session is exited. My configuration seems unuseable.