xfce4-terminal in fluxbox follows a different style/theme to xfce4-terminal in xfce

I am using xubuntu 18.04, but this question should preferably be applicable to all linux installations having both desktop managers "xfce" and "fluxbox". So I have started with xubuntu (i.e. ubuntu with xfce) but I have also apt install fluxbox as well. In "Xfce" I use xfce4-settings-manager to select a theme, the result for "Adwaita-dark" theme is shown below, for the "x-terminal-emulator" application: However, when I switch to "fluxbox": As you can see the window's menubar, as well as the preferences window do not obey the selected theme. This is a problem in "nautilus" file manager as well, but not for other applications (such as "Thunar" file manager, which works correctly) Does it perhaps not follow "gtk-2.0" styles ? For that I have set gtk-3.0 in "~/.config/gtk-3.0/settings.ini" [Settings] gtk-theme-name = Adwaita-dark gtk-icon-theme-name = SimplyGrey gtk-fallback-icon-theme = gnome gtk-font-name = Sans 10 # next option is applicable only if selected theme supports it gtk-application-prefer-dark-theme = true I have also setup QT to use GTK+ styles, but the issue persists. What am I missing here ?