from guizero import App, PushButton

def button_colour():
    if button.bg == "white":
        button.bg = "black"
        button.text_color = "white"
    else:
        button.bg = "white"
        button.text_color = "black"

app = App(title= "Countdown")
button = PushButton(app, text = "Countdown Button")
button.bg = "white"
button.text_color = "black"

button.repeat(1000, button_colour)

app.display()