Wrong background color transparency on Windows

[ikus060]

Describe the bug

On all platform the background color seams to get propagated to children. On all platform it seams to be alright except on Windows where children doesn't seams to inherit background. Reading about #767 , it might be an issue with background transparency ?

Steps to reproduce

"""
Opensource software to backup all your data.
"""
import toga
from toga.style import Pack
from toga.style.pack import COLUMN, ROW
import toga.platform

class MinarcaAgent(toga.App):
    
    def startup(self):
        white = toga.colors.WHITE
        dark = toga.colors.color('#0E2933')
        btn = toga.colors.color('#009FB9')

        main_box = toga.Box(style=Pack(direction=COLUMN), children=[
            toga.Box(style=Pack(direction=COLUMN, background_color=dark), children=[
                 toga.Label(text='MINARCA', style=Pack(text_align='left', color=white, padding=20)),
            ]),
            toga.Box(style=Pack(direction=COLUMN, padding=30, background_color=white), children=[
                toga.Label(id='lbl', text='Coucou', style=Pack(text_align='left', padding=20)),
                toga.Box(style=Pack(direction=ROW), children=[
                    toga.Button(text='I Love Python', on_press=self.click ,style=Pack(color=white, background_color=btn, padding=20)),
                ])
            ])
        ])

        self.main_window = toga.MainWindow(title=self.formal_name)
        # Remove useless toolbar
        self.main_window.content = main_box
        self.main_window.show()
    
    def click(self, event=None):
        lbl = self.main_window.widgets['lbl']
        lbl.text = 'Foo'


def main():
    return MinarcaAgent("Minarca Agent", "org.minarca.agent")

Expected behavior

I'm expecting something similar to Linux. Like this one.

Screenshots

Here the result on Windows:

Environment

• Operating System:
• Python version:
• Software versions:
  • Briefcase:
  • Toga:
  • ...

Logs

Additional context

No response