settings.decor_default_style (all fields optional, validated) seeds
_decorStyle once from the first config that arrives; every UI change of
the session default flows through one runtime method with a 1s debounce
and the ordinary serialized expected_rev write path. The built-in default
is stored as the absence of the key; a partial or garbage key falls back
per-field. decorStyleFromSettings/decorStyleToSettings are the single
snake_case<->camelCase conversion point.
User-Visible: yes
Issue: #377
Codex
10a1a26612