Enable specific CSS Code for Visitors and specific Roles

You can add custom css on your html header using wp_head hook action & checking your current User Role

add_action('wp_head', 'add_css_of_current_specific_user_role');
function add_css_of_current_specific_user_role()
{ 
    /*Check Current User is Login*/
    if ( is_user_logged_in() ) 
    {  
       $user = wp_get_current_user();
       /*Check Current User Role*/
       if (in_array('visitor', $user->roles)) { ?>

            <style>
              // Some CSS
            </style>

    <?php } 

    }
}