I wished to change the login theme within the KDE System Settings. However, clicking on apply gave the authentication error.
On the KDE System Settings chose Login Screen, under System Administratiion
On the Theme tab select one of the Themes for the login. At this point clicking on Apply gave the pop-up error message
Unable to authenticate/execute the action: (code 4)
To make the change to the login screen theme requires elevated permissions.
In the search box above the menu click within the search box and enter kdesu systemsettings.
I then clicked on the Run kdesu systemsettings in the box below.
The next screen presented is a password entry form. Enter the password for the root user.
Following on from this screen is the System Settings window. From there I was able to select the login and to add additional themes. Selecting the one I wished to use.
