I had the same problem a few days ago when I clicked onto night mode, but it seemed to work on another book inside my library so I thought it was something about that particular book that stopped it working properly.
Just now I thought I'd test out the night mode with a few books, I checked 1 imported book and two new books I just purchased over the last couple of days from Kobo and night mode isn't working on any of them properly. It seems like the white text, or rather the text that supposed to be white in night mode, isn't changing colours at all.
One particular book I checked has the chapter highlighted in a blue/green and that's the only text that's showing in night mode. So I reckon the text isn't changing in night mode when clicked so is disappearing into the black background.
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