lightbox-contact-form z-index

Hi Guys,
Recently, I download a plugin which call lightbox-contact-form. This plugin is really nice with a good-looking interface. However, some of its css doesn’t looks good for my theme. So I decide to modify it. However, this plugin using style.php. which totally not make sense to me. Can anyone help me to modify it?
Here is my testing page.
https://www.oasisled.ca/mytestingpage/
If you use width screen, you will see the image on the left side overlap the form. So I want to change its css which is
#acg_lbcf_div{
z-index: 1000;
}

Here is the PHP style code.
<?php

header(“Content-Type: text/css”);
header(“Vary: Accept”);

$url = $_SERVER[‘SCRIPT_NAME’];
$path = substr($url, stripos($url, “wp-content”));
$offset = $count = 0;
$strerror = “”;
$relpath = “”;
while (stripos($path, “/”, $offset)!==false && $count<=10) {
$offset = stripos($path, “/”, $offset)+1;
$count++;
$relpath.= “../”;
}
// include_once $relpath . ‘wp-config.php’;
include_once $relpath . ‘wp-load.php’;
include_once $relpath . ‘wp-includes/wp-db.php’;

global $wpdb;
$keys = “acg_lbcf_style”;
$wpdb->show_errors = true;
if (!is_array($keys)) {
$acg_mlm_keys = array($keys);
} else {
$acg_mlm_keys = $keys;
}
$q = “SELECT * FROM ” . WP_ACG_LBCF_CONFIG;
if ($key) {$q.= ‘ WHERE config_key=”‘ . $key . ‘”‘;}
$results = $wpdb->get_results($q);
foreach ($results as $row) {
$acg_mlm_vals[$row->config_key]=$row->config_value;
}
extract($acg_mlm_vals);
echo $acg_lbcf_style;

?>
I have no idea how to change its z-index.
Anyone can help me out please?
thanks a lots.