Updates one or more themes on a project and returns a process ID to track the work. Themes are identified by their stylesheet, the value returned as the theme key by GET /projects/{projectId}/themes. A theme already on its latest version, or one whose updates were excluded from the dashboard, is skipped; if none of the requested themes has an available update, the call returns 404 rather than an empty process. Repeating an identical call within a few minutes returns duplicate true instead of queueing the same work twice.