The "theme selector" switch at Admin Panel -> Settings -> Switches activates a selector where visitors can choose their preferred theme. The selector will only display when you have two or more themes loaded. The selector appears on every page since it's in the wrapper template. If you turn the theme selector off, registered members will still be able to change the theme by editing their profile but unregistered guests will be stuck with the default theme (or the mobile theme if their user agent is mobile).