Hi Sven, does changing the font and font size work for you?
I’m fighting with this too now, currently in Debian 11. Dark mode works, via the gtk-application-prefer-dark-theme setting, but not setting the font or font size.
I have the same settings.ini content as yours, except also the gtk-application-prefer-dark-theme=1 line, so:
[Settings]
gtk-application-prefer-dark-theme=1
gtk-font-name=Ubuntu 8
gtk-theme-name=Adwaita-dark
gtk-icon-theme-name=Adwaita
For me, the first line does control the dark theme (I can switch it on and off successfully), but the other lines, and in particular gtk-font-name does not take / is not applied at all.
Applications like Firefox and GEdit still have (relatively) huge font size.
Any hints about how to debug this further are much welcome.
I have ensured the gnome-settings-daemon is completely removed also, just in case.