You can change the font, size, shadow, and background color of subtitles and closed captions.
Go to your Netflix Account page.
From the Profile & Parental Controls section, select a profile.
Select Change for Subtitle appearance.
Choose your subtitle appearance settings.
Select Save.
Open the Netflix app on your device to start watching with your new subtitle appearance.
Begin playing a TV show or movie.
From the player controls at the bottom of the screen, select Settings to adjust subtitle Size and Style.
Choose your Size and Style preferences.
Resume playing the title.
From the lower right, tap My Netflix.
From the upper right, tap More.
Tap Manage Profiles .
Select the profile you want to edit.
Tap Subtitle Appearance .
Choose your subtitle appearance settings. The new settings will automatically save.
To reset, tap Restore Default Settings.
Go to Apple Support or visit the below articles for instructions:
Looking to turn on or off, or change your subtitle, caption, or audio language? Learn how to use subtitles, captions, or choosing your audio language.