I like that my desk has uniformity and that each element of it is related. That is why I am quite picky about icons, appearance and so on.
Something that made me think for a long time was that I could not change the icon of Kmail that comes out in the system tray (tray), and since I used a monochrome theme, it looked pretty ugly along with the rest.
For, Plasma I am using a theme called Aya, to which I have added the tray icons that belong to another topic called Helium, but unfortunately it is not completely complete.
It's not really the author's fault, but KDE, since certain applications do not use the icons found in /home/elav/.kde/share/apps/desktoptheme/ / icons /, which are the ones that appear in the system tray. This happens with Kmail, Akregator and others.
How do I solve it?
Well the only solution I found was the following. What you need to do is replace the icon Kmail found in the folder 22 × 22 / apps of the icon theme that we are using in KDE, by the icon of our preference.
Obviously, it is preferable that said icon is in format . PNG and is 22px wide and tall. It is also advisable to save the original icon, in case we want to recover it. And that's it. This is what my systray looks like now: