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.
Choose from the available built-in styles (Large Text, Classic, Default) to create or delete a style.
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.
Choose the settings for a new style or the one you’re editing.
In the bottom section, choose the colour and size to use for a specific font whenever it’s used in subtitles.
Allow video to override
Allow the video settings to override these settings.