Kivy: ScreenManager with custom transition clear color

shadercleartest.py

from kivy.app import App
from kivy.lang import Builder

root = Builder.load_string('''
#:import WipeTransition kivy.uix.screenmanager.WipeTransition
ScreenManager:
    transition: WipeTransition(clearcolor=(1, 1, 1, 1))
    Screen:
        name: 's1'
        Button:
            size_hint: 0.5, 0.5
            on_press: root.current = 's2'
    Screen:
        name: 's2'
        Button:
            size_hint: 0.5, 0.5
            on_press: root.current = 's1'
''')

class TestApp(App):
    def build(self):
        return root

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