Default Theme in Appearance & Welcome and the theme in Defaults for New
Members were stored separately, so a new member saw one on the sign-in page
and the other once the shared defaults applied. Sharing a theme now also sets
the Default Theme (when it is a built-in or a published custom theme),
changing the Default Theme updates the shared theme, choosing None removes it
from the shared set, and unpublishing a custom default drops it from both.
The shared version is untouched, so nobody is re-sent defaults.
An admin can store a snapshot of cosmetic settings as the server setting
member_defaults, with an allowlist of keys and value checks. Each member
receives the theme and effects part once, only where they never saved
that setting themselves or still have what an earlier set put there.
Apply to everyone raises a version so members get the set once more.
Server templates carry the snapshot without the version.
Amnibro