Currently there are 2 icons that seems to do the same: * settings-configure.svg * applications-system.svg Based on the name I see that they should not but the latter can be found for configuration in each applet to go to the settings. I tend to find the latter better than the first one, but this is another story. Reproducible: Always
applications-system.svg is a workaround icon for applications that used this categories icon in the toolbar. The correct icon to use in toolbars and buttons is settings-configure.svg Now as this is about the Plasma theme which has its own assets, separate from the icon theme, it should follow the icon theme.
Now it's on the other side, so this problem has been fixes.