Change Playback Captions preferences in iTunes on PC
In the iTunes app on your PC, use the Captions pane of iTunes Playback preferences to control how subtitles are styled.
To change these preferences, choose Edit > Preferences, click Playback, then click Configure.
Available built-in styles (Large Text, Classic, Default), and styles that you added.
Create a style or delete one that you added.
A new style inherits the settings of the style that’s selected when you click New. If you like many of the settings of an existing style, select it before you click New, then change just the settings you don’t like.
Settings for a new style, or the one you’re editing. If you want your setting to always be used, deselect its “Allow video to override” checkbox.
In the bottom section, you can set the color and size to use for a specific font whenever it’s used in subtitles.