Just add background_normal: '' to the TextInput in your kv.
I've set my TextInput's color to a diffrent one, even though - if you don't click on the TextInput you see a weird shadowed look on the TextInput. Blackish-grey at the top and white on the bottom. How do you get rid of this? I've tried foreground_color it doesn't work, even background_color - it doesn't work. How?
Kivy: how to get rid of the weird shadow look in TextInput?