timer2

from guizero import *

def promijeni():
    if button.bg == "black":
        button.bg = "white"
        button.text_color = "black"
        button.text = "White"
    else:
        button.bg = "black"
        button.text_color = "white"
        button.text = "Black"


app = App("Button background color")
button = PushButton(app, text="OK")
button.repeat(500, promijeni)

app.display()