Do templates, logos and watermarks apply to the guest login page too, or just the gallery?

Both — the guest access (login) page and the gallery are part of the same branded experience. When you preview a template you can check both sides before committing: the login page and the full-access gallery each get previewed separately. So from the moment a guest taps your link to the moment they download their last photo, it's your look, your logo, your watermark. That continuity is a big part of why delivery feels professional rather than like a random file-sharing service.