The style has the country polygons authored with transparency. It's meant to be used as an overlay on top of a basemap to add the color to the underlying map. So what you're seeing is expected.
You can see that effect in the online viewer:
www.arcgis.com/home/webmap/viewer.html?useExisting=1&layers=8566d3e326354e94877f3a9dd9c12ccb
Try and open the above link and change the basemap and you'll see how it adds the color to the underlying basemap.
You can turn off the lines on the background grid on the MapView.BackgroundGrid property or add a basemap.