I saw this today too, also after updating core discourse.
Looking at the theme component list and filtering by updates available showed me which one needed updating. The “Sidebar Theme Toggle” theme component needed updating, and selecting the circular arrows glyph updated it. The message on the admin landing page went away.
I don’t know why this particular theme component was not successfully being updated when core discourse is updated, and did not see anything in the log.
I also don’t know why the messaging didn’t list the theme component specifically.