Low resolution text in Python Kivy

I'm starting in Python and Kivy, but I'm not understanding why the low resolution of the generated objects. In a simple screen of username and password, it is possible to observe the text serrated, that is, with low rendering.

Is there a way to achieve better quality or is it a language limitation?

Follow files.

login.py

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

class Access(Widget):
    pass

class LoginApp(App):

    def build(self):
        return Access()

LoginApp().run()

login.kv

#:kivy 1.0.9

<Access>:
    f_username: username
    f_password: password
    GridLayout:
        pos: root.center_x-self.width, root.center_y-self.height/2
        col_default_width: 100
        rows: 2
        cols: 2
        Label:
            text: 'User Name:'
        TextInput:
            id: username
            multiline: False
        Label:
            text: 'Password:'
        TextInput:
            id: password
            password: True