You can change the font, size, shadow, and background color of subtitles and closed captions.
Using a browser, go to your Account page.
In the Profile & Parental Controls section, select a profile.
For some accounts, select Profiles, then choose a profile.
Select Subtitle appearance.
Choose your subtitle appearance settings.
Select Save.
Open the Netflix app on your device to start watching with your new subtitle appearance.
Open the Netflix app, then start playing a TV show or movie.
Pause Netflix, then select 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.
Open the Netflix app.
In the lower right, tap My Netflix .
In the upper right, tap the Menu .
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.