You can't change font, color, transperacy or any subtitle appearancy settings on Disney plus, but I just noticed a bug. If you install "Just player" from play store and change subtitles settings in Just player, same settings will apply to Disney plus.
In Just player long press subtitles button or settings button to find subtitles settings.
Watching HDR content on OLED in dark room without 50% transperacy for subtitles is eye burning torture
You don't need just player. Just player changes your android tv captions settings. There is no innate subtitles settings in just player, it uses android tv accessibility settings