And I guess the most obvious thing to test it with is mastodon.
And so:
theme color - #191B22
under page background color: #191B22 as well
sampled page top color: #282C37
So here the sampled color is better than theme color, but that one should be respected, so oh well.
More so if light theme is used - theme-color is still dark while both background and sampled colors are appropriately light...
And apparently there's one more layer to this.
https://developer.mozilla.org/en-US/ has theme-color set to white and yet Safari picks up sampled color: #1B1B1B
I'm honestly not sure why. Maybe it treats white theme-color as unset?..