Ideally you'd make a Flarum extension that extends the welcome banner and changes its template based on the value of
app.session.user, or move detection to the backend and conditionally display the banner based on a forum attribute.
app.session.user and conditionally loads a bit of CSS that shows/hide the banner. It might need a timeout to wait for Flarum to complete the boot and for
app.session.user to become available in the global scope.