function register_style() {
    if (is_page..something) {
        wp_enqueue_style('style', PLUGIN_URL . 'style.css');
    }
}
add_action('admin_enqueue_scripts', 'register_style');

function register_style() {
    wp_enqueue_style('style', PLUGIN_URL . 'style.css');
}

if (is_page..something) {
    add_action('admin_enqueue_scripts', 'register_style');
}