On some devices Topo, Refill, and Walkabout offline map styles have a blue background. In visual style this issue mostly affects Topo and Walkabout. Usually, these devices are older ones that have Android 5.1.1 or lower versions of Android and it is mainly occurring on Huawei devices. Unfortunately, this problem can't be solved as the Tangram ES library which GPX Viewer PRO uses for showing offline maps cannot work correctly on these devices as they have some bugs in their GPU implementations. You can find more about this issue on this GitHub issue: https://github.com/mapzen/android/issues/414.

Here is screenshot of the Walkabout offline map style and how it looks on the device which has this issue: