When selecting a dark theme like Greybird-dark or Adwaita-dark in Xubuntu's Appearance setting, GTK4 applications do not use the dark-theme and appear in light mode.
The only way to fix this was to enter this line into terminal:
echo "export GTK_THEME=Greybird-dark:dark" >> ~/.profile
Would it be possible to add an option in the Appearance tab that allows the user to toggle a switch for dark mode to make this easier ?