Styling Custom Meta Boxes?

Remove the wp_enqueue_style line from the above code and replace it with this:

add_action( 'admin_print_styles-post-new.php', 'portfolio_admin_style', 11 );
add_action( 'admin_print_styles-post.php', 'portfolio_admin_style', 11 );

function portfolio_admin_style() {
    global $post_type;
    if( 'portfolio' == $post_type )
        wp_enqueue_style( 'portfolio-admin-style', get_stylesheet_directory_uri() . '/css/portfolio-admin.css' );
}

It is better to put the styling for the admin panel in a separate file like in a theme_directory/css/portfolio-admin.css as the front-end styling could conflict with the admin styles.