The icon theme is already installed system-wide under usr/share/icons. I meant that the changed icons are then also used everywhere where they appear.
For example, I had tried to change the logout button in the whisker memu, but the change was then only in one place and not generally. So I now understand that it is more effective to change this directly in the icon theme. However, my idea of using the switch-off icon or another image instead is also problematic or confusing, because then I can no longer recognise which command I am actually triggering with it (only in the description), and there are then double-assigned icons. If so, I would have to use an image that has not yet been assigned to a specific command or reassign the command. This turned out to be more difficult than I thought, because there are often several variants of icons available (with and without arrow=shortcuts?), so I left it for the time being and will see whether I like the lively icon theme in the long term or whether it is a bit too restless (and on 14 inches the little images in the taskbar are very difficult to recognise and distinguish from each other).