main.py

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

class RootWidget(Widget):
    pass

class TestApp(App):

    def build(self):
        pass

if __name__ == '__main__':
    TestApp().run()

test.kv

#:set COLOR_HIGHLIGHT (0.788235294, 0.643137255, 1)
#:set COLOR_BACKGROUND (0.349019608, 0.082352941, 0.658823529)
#:set COLOR_BACKGROUND_A50 (0.349019608, 0.082352941, 0.658823529, .5)
#:set COLOR_BORDER (0.643137255, 0.160784314, 1)
#:set COLOR_BORDER_A50 (0.643137255, 0.160784314, 1, .75)
#:set COLOR_HIGHLIGHT_A50 (0.788235294, 0.643137255, 1, .5)

RootWidget:

<RootWidget>:
	FloatLayout:
		size: root.size
		pos: root.pos
		Button:
			pos_hint: {'x':.4, 'y': .4}
			size_hint: (.1, .1)
			text: 'rectangle'
			canvas.before:
				Color: 
					rgb: COLOR_BORDER
				Line:
					width: 2.
					rectangle: (self.pos[0], self.pos[1], self.size[0], self.size[1])
					joint: 'bevel'
		Button:
			text: 'ellipse'
			pos_hint: {'x': .6, 'y': .4}
			size_hint: (.1, .1)
			canvas.before:
				Color: 
					rgb: COLOR_BORDER
				Line:
					width: 2.
					ellipse: (self.pos[0], self.pos[1], self.size[0], self.size[1])
					joint: 'bevel'