import kivy
kivy.require('1.9.0')

from kivy.app import App
from kivy.uix.floatlayout import FloatLayout
from kivy.uix.image import Image

class FloatingApp(App):

    def build(self):
        return FloatLayout()

flApp = FloatingApp()

flApp.run()

<Botao@Button>:
    font_size: 32
    color: 1, 1, 1, 1
    size: 138, 138
    background_normal: 'bd.png'
    background_down: 'bd1.png'
    background_color: 0.88, 0.88, 0.88, 1
    size_hint: None, None

<FloatLayout>:
    Botao:

        text:"Botao"
        pos_hint: {"center_x": .5, "center_y": .5}