I haven't played Pokemon Go, but it's fine for apps to use their own custom theme. What's not fine is to use one OS's theme on another OS, like an Android app o

I haven't played Pokemon Go, but it's fine for apps to use their own custom theme. What's not fine is to use one OS's theme on another OS, like an Android app on WP. I see where you're coming from when you say that nice UX is not the most important thing, but a poor UX comes in the way of usability. The conventions users are used to no longer work, which comes in the way of using the app for its intended purpose. UI that doesn't fit might also cause users to pause and reorient themselves, again distracting them from their goal. For example, wall switches in India are on when pressed downward, as opposed to upward in the US. Either works, but if the switches are different from room to room in your house, it's confusing.