I notice that one can now (plasma 5.13) make the menu's transparent by fine-tuning the Breeze widget-style. It looks great! Specially with the new blur effect! Would it be hard to create the possibility to also apply this Breeze transparency/blur to the window-background so that we would get Oxygen-transparent kind of back but than better? https://store.kde.org/p/1005551/ .

At the very least there are other styles which do support Transparency, such as Kvantum and QtCurve (should be in the repos). You can find themes for them on store.kde.org; there are categories specifically for Kvantum and QtCurve themes.

Thank you, and of course you are right. But in my Neon User Edition I still can not install kvantum and qtcurve. With a little work I got them both to work but they both didn't impress me as the oxygen-transparent theme did.

I'm curious what else impressed you about oxygen-transparent that seems lacking from the transparent styles available for QtCurve or Kvantum?

Fyi, QtCurve should come with its own configuration dialog for tweaking all aspects of that style (usually accessed via System Settings -> Application Style -> Configure with QtCurve selected), including the transparency/opacity of the backgrounds.

If you're willing to invest some time in manually compiling and configuring, there's also StyleProject, which has a different feel to it, reminiscent of the Bespin theme that you could similarly manually compile and install for KDE/QT 4.