Functions – under the hood

While is_user_logged_in() can be used to determine the difference between a guest and a logged in (and therefore registered) user, switch_theme( $stylesheet ) alters actual database entries in the {$wpdb->options} table:

update_option( 'template', $template );
update_option( 'stylesheet', $stylesheet );
update_option( 'current_theme', $new_name );
if ( count( $wp_theme_directories ) > 1 ) {
    update_option( 'template_root', get_raw_theme_root( $template, true ) );
    update_option( 'stylesheet_root', get_raw_theme_root( $stylesheet, true ) );
} else {
    delete_option( 'template_root' );
    delete_option( 'stylesheet_root' );
}
update_option( 'theme_switched', $old_theme->get_stylesheet() );

Simple solution: Two stylesheets.

Therefore I wouldn’t recommend doing so. Simply switch “themes” – read: loaded stylesheets – on a user/guest basis

$stylesheet  = plugins_dir_url( __FILE__ ).'assets/';
$stylesheet .= is_user_logged_in()
    ? 'style-user.css'
    : 'style-guest.css;
wp_enqueue_style(
    'main-stylesheet',
    $stylesheet
    array( 'commons.css' )
    1.0
);

As you can see, I add a dependency of commons.css to the stylesheet. This would be another, previously registered/enqueued stylesheet that has all definitions that are shared between both.