Unsolved Smooth change of background image / PyQt6

[FoxyFox]

Hello everyone!
This is the part of my code that sets the background, an image taken from the root folder of the project. Yes, everything works. But... I've probably tried all the ways, and I can not make a function for example, which will smoothly change the background to white when I click on it. Help me please!

window = QMainWindow()
window.setStyleSheet("""
QMainWindow {
background-image:url(Images/default_image.png);
background-repeat: no-repeat;
background-position: center;
}
""")