You may want to state in your description of this that whichever theme is previewed becomes the “active” theme. You can then only revert back to your preferred active theme by choosing it again from the Preview page/post. It will not revert using admin/appearance - activate theme. Furthermore, if the theme shortcode is place in a page, and you preview a theme which does not show page-links, a user will have no way to return to the theme page (unless they have memorized the, hopefully friendly) permalink.
