WTH - Why does logging in with a fresh session mean HA resets to the default theme?

Why do I have to manually reset my theme whenever I log in to a new session in HA? Can this not be stored directly in HA rather than the browser? Unless there has been a change to this and I missed it?

It hasn’t changed, and I believe there are multiple topics in just the past month asking this and similar questions about moving stuff to the server, rather than per device.

e.g.: WTH Set dark theme as default on web browser