feat: govern accessible appearance overrides

docs/THEMING.md

@@ -22,22 +22,35 @@ inherits the result through semantic tokens without module-specific CSS.
   child scope; otherwise a tenant lock suppresses a personal override. The
   authenticated profile reports the effective palette, source, inherited
   palette, and lock state.
-- Tenant branding is a separate policy surface and must preserve contrast and
-  status semantics in both modes.
+- Advanced personal overrides are a separately governed surface. The system
+  must opt in, a tenant may inherit or block that decision, and palette locks
+  always suppress overrides. Changing either policy requires
+  `admin:policies:write` in addition to the owning settings permission.
 
 ## Palette safety and scope
 
 The Settings preview shows the chosen or inherited accent in every applicable
 light/dark preview before Save. Presets are checked for WCAG AA contrast in the
-theme contract. Arbitrary token overrides and branding import/export are not
-inferred from this preference and require their own governed follow-up.
+theme contract. When policy permits, the shared advanced editor can atomically
+override accent, surface, and semantic status pairs for both modes. Every
+foreground/background pair must meet WCAG AA contrast, and success,
+information, warning, and danger colors must remain distinct. Invalid stored
+documents fail closed and are not partially applied.
+
+Import and export use the exact versioned JSON schema `schema_version: "1"`.
+Both `light` and `dark` must contain every supported token exactly once as a
+six-digit hex value. Import changes only the local draft; Save persists the
+whole document. Removing overrides returns to palette and policy inheritance.
+The system default is disabled so upgrades do not unexpectedly admit arbitrary
+branding. Tenant `null` means inherit, `false` blocks, and `true` is accepted
+only while the system permits overrides.
 
 Do not introduce fixed foreground/background colors in a module merely to make
 one mode look correct. Add or reuse a semantic Core token, then define both
 light and dark values. Bitmap content and externally authored HTML are exempt,
 but their surrounding controls must still use the shared tokens.
 
-`npm run test:theme-contract` verifies root mode/palette behavior, preset
-contrast, and representative
+`npm run test:theme-contract` verifies root mode/palette behavior, preset and
+custom-override validation/application, and representative
 Campaign, Calendar, Files, and Mail token consumption. The check runs before a
 production WebUI build.