login_screen.kv

<LoginScreen@LoadScreen>:
    id: login_screen
    name: "login_screen"

    MDTextField:
        id: password_textfield
        name: 'password_textfield'
        hint_text: "Enter Password"
        helper_text: "(or click on forgot forgot)"
        helper_text_mode: "persistent"
        pos_hint: {'center_x': .5, 'center_y': .5}
        size_hint_x: None
        width: 300

    MDRectangleFlatButton:
        id: forgot_password_button
        name: 'forgot_password_button'
        text: "Forgot Password"
        pos_hint: {'center_x': .4, 'center_y': .4}
        theme_text_color: 'Custom'
        text_color: (100 / 255.0, 100 / 255.0, 100 / 255.0, .8)
        on_release: root.show_data()

login_screen.py

from utils import load_kv

from kivymd.uix.screen import Screen

from kivymd.uix.dialog import MDDialog
from kivymd.uix.button import MDFlatButton
from kivy.lang import Builder


class LoginScreen(Screen):

    def show_data(self):
        if len(self.password_textfield.text) < 1:
            return

        self.password_dialogue = MDDialog()
        self.password_dialogue.text = f'You entered: {self.password_textfield.text}'

        self.password_dialogue_button = MDFlatButton()
        self.password_dialogue_button.text = 'Close'
        self.password_dialogue_button.on_release = self.close_dialogue

        self.password_dialogue.open()

    def close_dialogue(self, obj):
        self.password_dialogue.dismiss()


load_kv()

main.kv

#:import TitleScreen title_screen.title_screen
#:import LoginScreen login_screen.login_screen

ScreenManager:
    id: manager

    LoadScreen:
    TitleScreen:
    LoginScreen:

<LoadScreen>:
    id: load_screen
    name: "load_screen"

    Label:
        id: load_screen_label
        name: "load_screen_label"
        text: "Loading..."

main.py

import os
import sys
from pathlib import Path

from kivy.lang import Builder
from kivymd.app import MDApp
from kivy.uix.screenmanager import Screen

os.environ["KIVY_PROFILE_LANG"] = "1"

if getattr(sys, "frozen", False):  # bundle mode with PyInstaller
    os.environ["UNNATURAL_SELECTION_ROOT"] = sys._MEIPASS
else:
    sys.path.append(os.path.abspath(__file__))
    os.environ["UNNATURAL_SELECTION_ROOT"] = str(Path(__file__).parent)

os.environ["UNNATURAL_SELECTION_ASSETS"] = os.path.join(
    os.environ["UNNATURAL_SELECTION_ROOT"], f"assets{os.sep}"
)


class LoadScreen(Screen):
    #lets do Database and Animation stuff here
    pass


class UnnaturalSelectionApp(MDApp):

    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self.theme_cls.theme_style = "Dark"

    def build(self):
        return Builder.load_file(
            os.path.join(
                os.environ["UNNATURAL_SELECTION_ROOT"], 'main.kv'
            )
        )

    def change_screen(self, screen_name):
        self.root.current = screen_name

    def on_start(self):
        self.change_screen('title_screen')


app = UnnaturalSelectionApp()
app.run()

title_screen.kv

<TitleScreen@LoadScreen>:
    id: title_screen
    name: "title_screen"

    MDLabel:
        text: "Unnatural\n    Selection"
        font_style: "H1"
        theme_text_color: 'Custom'
        text_color: (100 / 255.0, 100 / 255.0, 100 / 255.0, .6)
        pos_hint: {'center_x': .5, 'center_y': .5}
        halign: 'center'

title_screen.py

from utils import load_kv

from kivy.uix.screenmanager import Screen
from kivy.app import App


class TitleScreen(Screen):

    def on_touch_up(self, touch):

        app = App.get_running_app()
        app.change_screen('login_screen')


load_kv()

utils.py

import inspect
from os.path import extsep, exists, splitext

from kivy.lang import Builder


def load_kv():
    '''This magical function lookup module name, and load the kv file
    with the same name (in the same directory)
    https://gist.github.com/tshirtman/d8423695fe821440b65688b8dbd22b0c
    '''
    filename = inspect.currentframe().f_back.f_code.co_filename
    f = extsep.join((splitext(filename)[0], 'kv'))
    if exists(f) and f not in Builder.files:
        Builder.load_file(f)