Used a little PHP to set the theme-color meta tag per page. The compact tab bar in Safari now always matches the page color across my site.

Lovely detail, congrats Charlie! Yet, for some reason it doesn't seem to work for me. I'm using Safari v 17.1 (19616.2.9.11.7).

That would be my fault. I ended up removing the behavior because I moved some of the top elements on the pages so they are no longer flush with the top bar. The color in the top bar is also only available in the 'Compact Tab Bar' mode. Thanks for checking out the site!