[Python] Kivy Using a ScreenManager from kv file

I'm writing a kivy program/game. I've made one before, but was only 1 screen in total. I've started developing a new program with ideas of implementing a Screen Manager. I managed to build the Screen Manager inside of the python file, but it wasn't suitable for my long term usage, and I wanted to try my hand at a Screen Manager from the .kv file.

Any guidance will be greatly appreciated, not sure what I missed. I looked at this questionv(Kivy - Screen Manager - Accessing attribute in other class), and pieced together what I thought was right, but still can't get a screen to load - I feel this is a simple answer and I'm being blind...

main.py;

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen, FadeTransition


class MenuScreen(Screen):
pass

class NewGameScreen(Screen):
pass

class LoadGameScreen(Screen):
pass

class ASCIILifeApp(App):
pass

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


ASCIILife.kv;

#: kivy 1.9
#: import ScreenManager kivy.uix.screenmanager.ScreenManager
#: import Screen kivy.uix.screenmanager.ScreenManager
#: import NewGameScreen screen
#: import LoadGameScreen screen

ScreenManager:
id: screen_manager
#transition: FadeTransition()
MenuScreen:
id: menu_screen
name: 'MenuScreen'
manager: 'screen_manager'
NewGameScreen:
id: newgame_screen
name: 'NewGameScreen'
manager: 'screen_manager'
LoadGameScreen:
id: loadgame_screen
name: 'LoadGameScreen'
manager: 'screen_manager'

<MenuScreen>:
BoxLayout:
orientation: 'vertical'
Label:
text: 'ASCII Life'
font_size: 50
Button:
text: 'New Game'
font_size: 30
on_release: app.root.current = 'newgame'
Button:
text: 'Load Game'
font_size: 30
on_release: app.root.current = 'loadgame'
Button:
text: 'Settings'
font_size: 30
on_release: app.root.current = 'something'
<NewGameScreen>:
BoxLayout:
orientation: 'vertical'
BoxLayout:
orientation: 'vertical'
Label:
text: 'Game Length in Days'
font_size: 30
BoxLayout:
orientation: 'horizontal'
ToggleButton:
text: '100'
state: 'down'
group: 'newgame_days'
font_size: 30
ToggleButton:
text: '200'
group: 'newgame_days'
font_size: 30
BoxLayout:
orientation: 'horizontal'
ToggleButton:
text: '365'
group: 'newgame_days'
font_size: 30
ToggleButton:
text: '3650'
group: 'newgame_days'
font_size: 30
ToggleButton:
text: 'Unlimited'
group: 'newgame_days'
font_size: 30
BoxLayout:
orientation: 'vertical'
size_hint: (1, .5)
Label:
text: 'Difficulty (Score Multiplier)'
font_size: 30
BoxLayout:
orientation: 'horizontal'
ToggleButton:
text: 'Easy (x1)'
state: 'down'
group: 'newgame_difficulty'
font_size: 30
ToggleButton:
text: 'Medium (x2.5)'
group: 'newgame_difficulty'
font_size: 30
ToggleButton:
text: 'Hard (x5)'
group: 'newgame_difficulty'
font_size: 30
<LoadGameScreen>:
BoxLayout:
orientation: 'vertical'
Label:
text: 'load a game'
font_size: 30
Widget:
canvas:
Ellipse:
pos: self.pos
size: self.size
BoxLayout:
Button:
text: 'Menu'
font_size: 30
on_release: app.root.current = 'menu'
Button:
text: 'text'
font_size: 30


Edit: Truncated kv file

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