To enable closed captioning or subtitles for the deaf and hard of hearing:
Go to Settings > General > Accessibility > Subtitles & Captioning.
Tap the switch next to Closed Captions + SDH.
iOS 7 also allows you to customize the style of the captions or subtitles to better suit your needs.
Go to Settings > General > Accessibility > Subtitles & Captioning > Style.
You can choose one of the predefined styles: Default, Large Text, or Classic.
To create your own style, select Create New Style… You can change the font, size, and color of the text along with the color and opacity of the background.
To preview your new style in full screen, tap .
Tap Save, at the top of the window, when you are happy with the new style. This style will be the default style for captions and subtitles.