Serviio

I have some mp4 video files with the subtitles in a separate vtt file. The subtitles display OK when I play the video from Serviio, but they're a) in too big a font size, and b) not close enough to the bottom of the screen, so that they get in the way of viewing the action. If I play the same mp4 and vtt on my PC with VLC, then the subtitles appear in a more suitable smaller size and are positioned closer to the bottom of the screen. How can I change the size and position of the subtitles in Serviio?
I'm running Serviio 1.10.1 on https://19216801.onl/ https://routerlogin.uno/.

I don't see those spam links at all.

Stealth achieved.