<p>I have read the documentation of <code>gtk_widget_modify_font()</code> which is used in <code>ui_widget_modify_font_from_string()</code>. The function is deprecated and the gtk documentation suggest to use <code>gtk_widget_override_font()</code>. But that also has a deprecation remark:</p>
<blockquote>
<p>gtk_widget_override_font has been deprecated since version 3.16 and should not be used in newly-written code.</p>
<p>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.</p>
</blockquote>
<p>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).</p>

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