org.freedesktop.appearance.color-scheme is supported in a weird way

There are currently two implementations of org.freedesktop.appearance.color-scheme: one in the xdgdesktopportal platformtheme and in in qgenericunixthemes.cpp's QGnomeTheme, this leads to the following behavior:
1. QGnomeTheme subscribes but actually never uses the signal result and it never gets initial value.
2. If the app is in flatpak or snap, org.freedesktop.appearance.color-scheme is used but it doesn't update the value on the fly (as the implementation inside xdgdesktopportal doesn't support that)
3. In all the other cases (not flatpak or snap), the standard is not used

Expected behavior: standard is preferred whenever returns a non-unknown value and subscribes to updates whenever the standard is used.