I have local and remote access to my HA installation. Seems like the theme selection in the user profile depends on the way HA is accessed. When I connect with the same user either way it looks like the theme configuration in the user profile is per path to HA. I can change the theme for the user when connected locally - this does not affect the theme that is being configured and used when connecting through the external connection. Again, it’s the same user.
I wonder if this is intentional or not. Seems odd that the theme depends on the access path to HA.