WordPress Plugin greying out disabled buttons

grey-out-disabled.php

<?php

/*
Plugin Name: Grey out Disabled
Plugin URI: https://gist.github.com/mcguffin/7cbfb0dab73eb32cb4a2
Description: Grey out disabled submit buttons
Author: Jörn Lund
Author URI: http://github.org/mcguffin
Version: 0.0.1
*/

function recapcha_grey_out_disabled_button_css() {
	// inject script only in frontend
	global $pagenow;
	if ( ! is_admin() && $pagenow !== 'wp-login.php' ) {
?><style type="text/css">
input[type='submit'][disabled],button[disabled] {
	color:#999;
	background-color:#ccc;
}
</style><?php
	}
}

add_action('wp_print_scripts','recapcha_grey_out_disabled_button_css');