Sigil on Gnome on my Arch Linux system changes with the system theme. I'm also using Sigil 2.4.2 with Qt6.8.2. That said, I'm pretty sure that Gnome has a prefer dark or prefer light setting (somewhere) that defaults to prefer dark. Sigil should honor that setting.
There's no need to go manually diddle the preview_dark_in_dm value in either PageEdit or Sigil's INI file. They both have a GUI preference setting to change that. Kevin covered that.
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