As of the release of KDE 4.10 none of my plasma-themes show a background color of the system tray if there is one.
The system tray background is in all cases the same as the panel background color. Can I bring the orriginal background color back?

I suspect this possibly may have been caused by the rewrite to QML of the System Tray applet.
If this is the case, it probably is possible to restore the functionality (as QML can be edited on your system without needing a recompile) however i'll need to check with the developers.

It may be that the different background functionality is being dropped.

Wow, such a quick reply!
I am a relative noob. But of course I looked for bugs. I couldn't find any.

I would like it if this functionality returned.
Thank you.

by coincidence I noticed that the file "panel-background.svgz" in the directory /opaque/widgets of the plasma theme is not taken in account anymore.

Most of the time the panel is rendered in an unpredictable way in opaque. If there is a gradient in the opaque panel it is not rendered.
Is this functionality obsolete too?

I've checked with the developers, and this is indeed a consequence of the port to QML.

While the functionality can be restored, it is an option they would rather not restore if possible. As such it is not likely to get reimplemented within the QML applet by the developers themselves. If you are interested in it, I would suggest looking into the responsible QML and suggesting a patch.

Thank you very much for taking the afford. Now I know how it is.