According to the documentation here the UserInfo (and global variable) should indicate which theme the user is currently using. This works fine, except that in mobile devices, the theme always indicates SF1 (Theme4t). Even if the user is using a mobile browser (Safari on iPad for instance) and is in the full Salesforce site in classic view. This makes it impossible to treat the SF1 app or lightning vs classic UI differently on any mobile device. I've tested this on iPad, iPhone and Android (Chrome browser).

Is this by design? If so, this should be noted in the documentation.