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Re:Safari 15 on Mac OS, a user interface mess by sirn on Saturday June 19, 2021 @11:02AM (#27561016)
They do use theme-color and only fallback to either page background color or header background color when theme-color is not present[1][2]

[1]: https://files.grid.in.th/z3ox7o.jpg

[2]: https://developer.apple.com/videos/play/wwdc2021/10029/

Re:Safari 15 on Mac OS, a user interface mess by petepete on Saturday June 19, 2021 @12:03PM (#27561658)
Ah, thanks for clarifying.