Reproducer and a screenshot illustrating the issue are attached.

In light mode, everything looks fine - the ComboBox has a white background and black text inside. But in dark mode, ComboBox still has white background and text is totally invisible (See screenshot. I assume text becomes white as OS theme changes?).

I blame the fact that QML Window is not styled. Since with ApplicationWindow, everything looks perfect in both light and dark mode. Specifically, I think the good "updating all its children including popups" practice only exists in ApplicationWindow implementation but not Window implementation:
https://github.com/qt/qtdeclarative/blob/dev/src/quicktemplates/qquickapplicationwindow.cpp#L132

import QtQuick
import QtQuick.Window
import QtQuick.Controls.macOS

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    ComboBox {
        model: ["ONE", "TWO", "THREE"]
    }
}