Argh; it seems like some Qt styles simply ignore a text color that I set for a QRadioButton. I experimented with different Qt styles (set with "qt5ct" on my system), and at least the "Adwaita" and the "Gtk2" widget style ignore the text color for QRadioButtons; they always use their default text color. My default widget style is "Fusion", and with that one it works (see my screenshots); also with most others that I tried: "Cleanlooks", "Motif", "CDE", "Windows". By default, Xfce used "Adwaita" IIRC, and I would expect a "Gtk3" theme for GNOME to behave the same way as the "Gtk2" theme. So if this depends on the widget style, I don't know what we can do about it; if the widget style simply refuses to use the palette that we set, what else can we do?