I'll take a peek. But I'm pretty sure the dark theme detection was working correctly last time I looked--especially with FORCE_SIGIL_DARKMODE_PALETTE. Some of the other "dark" system themes... maybe, but not the inherent one we created for Sigil. But all bets are off with Qt6. I don't know that I ever even tried to customize CodeView colors using Qt6.
But it sounds as if he got something working there after all.