I have read the documentation of ```gtk_widget_modify_font()``` which is used in ```ui_widget_modify_font_from_string()```. The function is deprecated and the gtk documentation suggest to use ```gtk_widget_override_font()```. But that also has a deprecation remark:
gtk_widget_override_font has been deprecated since version 3.16 and should not be used in newly-written code.
This function is not useful in the context of CSS-based rendering. If you wish to change the font a widget uses to render its text you should use a custom CSS style, through an application-specific GtkStyleProvider and a CSS style class.
So I assume as soon as there is a CSS file we would need to apply a CSS style generated from the preferences settings to let the changes have any effect in this case (not tested - unconfirmed).