Yeah there is option to change text box opacity in game, Just follow this instruction you will find out in game
1) In the game menu screen you find Preference button, just click on it.
2) After clicking you will directed into co-menu in that you will find Transparency under transparency there is a slide bar called Dialogue Box just slide the button from 0 - 100 (default 50) where opacity of dialogue color change from transparent to Fully black colored dialogue box and Thanks for Downloading StH.
