Hello there,
I had the same issue a while ago.
So I did some digging and found that you have to edit the file in
/usr/share/plasma/shells/org.kde.plasma.desktop/contents/osd/
rather then in the look-and-feel folder. Please note that it is in the root folder and the file does get over written when kde gets an update, so you do have to redo it.
Hope this helps
