I changed from Gnome to KDE, really like it so far, but I want to change my notifications icons, like the increase/decrease volume, increase/decrease lighting on keyboard, toggle touchpad. I really liked the ones on gnome, but don't seem to find a solution on how to change them. An example of what I am looking to have https://ubuntu-mate.community/t/how-can-i-disable-popup-notification-for-volume-control/6968
KDE Plasma OSD icons
Plasma OSD is using icons from the plasma desktop theme - volume OSD icons are in audio.svgz:
Plasma theme details: https://techbase.kde.org/Development/Tutorials/Plasma5/ThemeDetails
Plasma desktop theme can be changed from the KDE System Settings > Workspace Theme > Desktop Theme
Get some Icon packs. That will change the icons.
Good one to start with could be Paper.