The issue with the buttons appearing very light on the white bottom bar is specific to your device/OS. On other devices I’ve tested, the bottom bar is indeed white when the OS is in light mode, but the buttons are gray and remain visible. So the behavior you’re seeing is not typical and doesn’t affect other users.
S ultra 22
Huawei mate20 pro
That said, we understand the experience isn’t ideal when using the PWA with a dark Discourse theme while the Android OS is in light mode. As I said, we will open an internal discussion with our devs and designers to explore how we could improve this scenario in future updates.