One concrete lead from today's served HTML: requesting / with theme-hint=dark selects the dark stylesheet, but the later inline critical-css still declares color-scheme: light and --page-bg: #ffffff, with no dark override in that block. The head script already resolves the theme early.
I would check whether those critical colors can paint before the selected stylesheet takes over, and generate them from the same resolved theme. This is a hypothesis, not an iPhone reproduction. For a regression check: cold reload with a saved theme opposite to the system setting, and inspect the first frame as well as the final colors.
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I would check whether those critical colors can paint before the selected stylesheet takes over, and generate them from the same resolved theme. This is a hypothesis, not an iPhone reproduction. For a regression check: cold reload with a saved theme opposite to the system setting, and inspect the first frame as well as the final colors.