Kivymd full app

from kivymd.app import MDApp
from kivy.uix.screenmanager import Screen, ScreenManager

class MainWindow(Screen):
    pass
class SecondWindow(Screen):
    pass
class WindowManager(ScreenManager):
    pass
sm = WindowManager()
screens = [MainWindow(name="main"), SecondWindow(name="second")]
for screen in screens:
    sm.add_widget(screen)

class MyApp(MDApp):
    def build(self):
        return sm

if __name__ == "__main__":
    MyApp().run()