Put black background in subtitles (accessibility)

From your parallel post in the tips & tricks / how-to thread I take it you already found how to change the font size, weight, color and borders by editing the mpv.conf file in the Plex Media Player / Plex for Windows / Plex HTPC app config directory?

Given Plex Media Player is no longer officially supported, I take it this is about Plex for Windows or Plex HTPC?