“…when the user has “use default theme” configured in their user settings.”
I have (in German)…“Standardthema verwenden” which is, I assume what you mean. Selected this in user settings, but when I log out and back in, same problem…it’s not the standard HA theme but another theme in my list (dark mint). Also cleared browser cache…
But the fact that it always reverts to “dark mint”…maybe I have to delete this theme in my list.
So, deleted all other custom themes in my drop-down list and set my default list to my only custom one. The only problem now is that the dark mode is not saved.