@Barb Bowman, @Johnny55,
I just tried another test on this machine (Build 26100.712), as suggested by Kyle Pflug on your earlier post.
On Edge Settings appearance I set it to Dark and then the local Copilot App opened also in Dark mode appearance.
Looks like there is more than one way to obtain the same result. 😎😂🤷♂️
It also works with System default, assuming that the system is set for Dark color. 👍
Edit:
Got this error again when changing the color appearance. This is for MS to figure out. 🤷♂️