Uses the same method to calculate that as the Colors KCM. (https://invent.kde.org/plasma/plasma-workspace/-/blob/master/kcms/colors/filterproxymodel.cpp#L106)
Test using:
$ qdbus org.freedesktop.portal.Desktop /org/freedesktop/portal/desktop org.freedesktop.portal.Settings.Read "org.freedesktop.appearance" "color-scheme"1 = dark, 2 = light