kv_login take two

login.kv

<UsernameWidget>:
    BoxLayout:
        size: root.size
        pos: root.pos
        orientation: "horizontal"
        
        Label:
            text: "Username:"
    
        TextInput:
            font_size: 80,
            size_hint_y: None,
            height: 110
            
            
<PasswordWidget>:
    BoxLayout:
        size: root.size
        pos: root.pos
        orientation: "horizontal"
        
        Label:
            text: "Password:"
            
        TextInput:
            font_size: 80,
            size_hint_y: None,
            height: 110
            password: True
            

<LoginScreen>:
    BoxLayout:
        orientation: "vertical"
        size: root.size
        pos: root.pos
        
        UsernameWidget:
        PasswordWidget:
        
        Button:
            text: "Login"
            on_press: self.on_login

login.py

from kivy.app import App
from kivy.uix.widget import Widget

class UsernameWidget(Widget):
    pass

class PasswordWidget(Widget):
    pass

class LoginScreen(Widget):
    pass

class LoginScreenApp(App):
    def build(self):
        screen = LoginScreen()
        return screen

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