Address review follow-ups on PR #3568:
- Extract the controller's withoutThemeFields and applyDefaultTheme helpers from a private class field into a standalone module (controllers/statusPageThemeFlag.ts) so they can be tested without a controller harness; the controller is now a one-line consumer
- Add 9 unit tests covering both helpers (test/unit/controllers/statusPageThemeFlag.test.ts): no-mutation guarantees, partial inputs, all-fields-preserved verbatim, override-with-defaults semantics, and "page has no theme set yet" — the flag-off path is now exercised in both directions
- Expand the comment over withoutThemeFields to spell out why the strip is silent rather than a 400: this is an ops kill switch, not API semantics; clients shouldn't need to know whether themes are disabled today
Two caveats called out in review that this commit does NOT change:
- Mongoose `default:` directives only inject during hydration, not on partial updates. Legacy documents updated with `findOneAndUpdate` still won't have `theme` / `themeMode` written to Mongo. They render as defaults on read because the schema injects them — functionally equivalent, but the DB will hold a mix of present and absent fields indefinitely. Acceptable per the original commit intent.
- docs/plans/status-page-themes.md (304 lines) covers both backend and frontend work for the themes feature. It lives in this PR as the design artefact that drove both branches; moving it for purity would break the "where did this come from?" trail. Leaving it in.
gorkem-bwl
d80d939bd8