The Viki app is designed to follow the android system settings with respect to subtitles, so the appearance of subtitles will be customisable to the extent that the android TV system settings allow. The same goes for Amazon fire TV devices. Although the settings are found in a different place to what I have indicated below as the operating system is slightly different on fire devices. It would be helpful if Viki made the fact that you can customise subtitle appearance a bit more obvious.
The settings are usually found in
Settings > device preferences > accessibility > captions
Hope this helps 