Beeware Toga Selection widget: unable to change text size

Question

In my Beeware app I am trying to use the Selection widget according to https://toga.readthedocs.io/en/latest/reference/api/widgets/selection.html setting the value of the the font_size parameter. To test it I use the simple script attached below. The widget works correctly, but the font size remains small and is not affected no matter what value I set. Any suggestion or hint how to make it work will be much appreciated

import toga
from toga.style import Pack
from toga.style.pack import COLUMN, ROW


class HelloWorld(toga.App):
    def startup(self):
        self.selection=toga.Selection(style=Pack(font_size=30),\
                items=["title", "message","sound"],on_select=self.func1)
        main_box = toga.Box(style=Pack(direction=COLUMN))
        main_box.add(self.selection)

        self.main_window = toga.MainWindow(title=self.formal_name)
        self.main_window.content = main_box

    def func1(self,widget):
        print(self.selection.value)
        return 

def main():
    return HelloWorld()