plugin css is not being applied to the page?

In wp_enqueue_style()‘s 5th parameter is not boolean. Use following code:

function libload()
{
    wp_enqueue_style('style_file' , plugin_dir_url(__FILE__).'lib/plugin_styles.css');
    wp_enqueue_script('js_file',  plugin_dir_url(__FILE__).'lib/plugin_js.js', array(), null, true);
}

add_action('wp_enqueue_scripts','libload');