SystemPalette not correct on Linux / Breeze dark?

[heikotietze]

I'm running Arch Linux with KDE/Breeze Dark and expect my own Qt/QML application to blend smoothly into the system theme. Fact is that my ApplicationWindow has a bright background unless I do something like

background: Rectangle {
    anchors.fill: parent
    color: config.myPalette.window
}

which works but not for the menu, for example. Could do the same here and for every widget but that's very tedious.

Tried with QApplication vs. QGuiApplication, which has no effect, and with

app.setPalette(app.style()->standardPalette());

which updates the colors but into a bright mode.

Using a different style but Fusion does not change the colors neither setting palette/Window = System. I removed the qtquickcontrol now completely.

Everything works fine on other OS, at least the dark mode on macOS looks beautiful (Android is bright, Windows does its own thing). What can I do?