The theme preview flow is the WordPress path used to inspect a theme before it is activated. It lets an administrator load a theme in a temporary viewing mode so the site can be tested without fully switching the live appearance. Because this flow runs inside the admin interface, it is part of a trusted workflow and can influence privileged actions.
In security terms, the preview flow matters because it sits close to theme installation and activation. If user-controlled data is not handled safely, a preview URL or related UI state can be abused to trigger the wrong action, such as a click on install or an unintended theme change. Defenders should treat preview parameters as untrusted input, apply strict capability checks, validate nonces, and escape values before they reach browser selectors or admin scripts. A safe preview should never become a deployment path.