I'm running kubuntu 18.04, primarily using breeze dark, KDE Plasma 5.12.6 and nVidia drivers 410.73.

I have for a long time tried to make the panel at the bottom semi transparent and failed. Eventually I found out how to do it by modifying the panel-background.svgz with InkScape, it is located in desktoptheme/default/translucent and not desktoptheme/breeze-dark/... .

The panel background is inherently dark/black and when I change the transparency, it fades to white the more transparency I choose. That is when I use OpenGL 2.0 or OpenGL 3.1 as compositor.

Then I use Xrender as compositor, it works as it should and I get transparency towards the background desktop. However, in Xrender the popups from when clicking on the tasks on the panel are misaligned with the tasks and some tasks don't even show up on the panel. Also the panel clock doesn't work either. This is working properly in OpenGL compositors.