The black background box for subtitles is visually jarring compared to the polished look of the rest of the UI - I understand the need to make the white text easily visible, but slapping a big dark box around it is like looking at an MS Paint cutout pasted into a carefully photoshopped sunset.
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I haven't seen the subtitles, but from what you are saying the black box should be an additional setting, along with text on no background.