Comment # 9 on bug 970653 from Raymond Wooninck

Donald, 

As Wolfgang already mentioned, the icons displayed in the systray as taken from
the Plasma theme set in "Desktop Theme". However if that theme doesn't have
icons defined, then it falls back to the default. If you look at the Oxygen
Desktop theme, then you will find that it doesn't define any icons that can be
used for plasmoids and/or the systray itself. Try changing to another theme and
you will see that with certain themes the icons will change. This is not a bug,
but purely how the fallback scenario works. I am sure that you would agree that
the fallback is better than to display just empty spaces as that there are no
icons. 

The Oxygen-icon-theme packages are just the applications icons, etc. They do
not contain any icons for plasmoids, etc. They just have the application icons
and icons to display folders, etc. 

If you feel this is a missing feature, then you should submit a feature request
for the oxygen desktop theme to also include icons. This feature request should
be done at http://bugs.kde.org.

What you are describing is not a bug, but just a missing feature of the chosen
Oxygen Desktop Theme. A feature that was never implemented.  Keep in mind that
the same logic was used in KDE4, but the only difference was that back then
Oxygen was the standard. And this now has changed to Breeze.