I think it's still generally the case that Kivy apps tend to use the default (clunky) theme or something more custom, but there has recently been some user activity around alternatives, e.g. a set material design themed widgets (https://github.com/kivymd/KivyMD). I'm not sure if any apps using this have been published on the Play store.

We've been discussing how to include more unified themeing capabilities in Kivy itself, ideally as a big new feature in the upcoming 2.0 release.

Edit: I wrote a post at http://inclem.net/2016/01/15/kivy/kivy_android_app_showcase/ about some of the best published Kivy apps including themeing considerations, although they don't aim for a standard Android UI either. I see in your edit you've already seen this though.