The only way I know to do this, is by creating a new theme that uses the font.
But for some reason the item button disappears from the toolbar when doing this.
So I think you would have to edit your items from script, maybe there is an other way but I have no idea how to work with themes.
What I do to avoid this is creating the theme in the
ready() function only when my app is running.
$OptionButton.theme = Theme.new()
$OptionButton.theme.default_font = DynamicFont.new()
$OptionButton.theme.default_font.font_data = load("res://your_font_file")
That way you can still easily edit your items from the editor.