How to switch theme if the current user is admin?

You are sort of doing this in a roundabout way. WordPress has a function called switch_theme():

add_action( 'setup_theme', 'switch_user_theme' );
function switch_user_theme() {
  if ( current_user_can( 'manage_options' ) ) {
    switch_theme('twentytwelve');
  } else {
    switch_theme('twentythirteen');
  }
}

The argument is the directory of the theme you want.

I can’t help but think this is a bad idea though. Surely you can do what you need without switching themes constantly?