Thank you so much for the feedback!
And my apologies about that! I thought that without the textbook, it looks a bit cleaner like subtitles in a movie - of course the tradeoff is that the white color of the text against the background will be hard to read as you said. That was why I decided to include an option to adjust the opacity and also, padded the text or added some shadow beneath the text to make it more visible.
But I will keep this in mind! Was the maximum textbox opacity still too weak for you? I can adjust it to make the maximum opacity darker if that's the case!