Hi,
Here is better solution and will be included in next release, so u will not loose map custom style after upgrade.
Plz download attached zip, extract it and upload map.js in
your wordpress >  
wp-contentpluginsgeodirectorygeodirectory-functionsmap-functionsjs
after uploading js in correct folder, plz put this snippet in ur theme’s functions.php file 
add_action('wp_footer'  , 'geodir_set_custom_map_style');
function geodir_set_custom_map_style()
{
?>
<script>
	
	var geodir_custom_map_style =[
					// Google Map Styles Starts Here
					{
					"stylers": [
					  { "invert_lightness": true },
					  { "saturation": -100 }
					]
					},
					// Google Map Styles End Here
					{
						featureType: "poi.business",
						elementType: "labels",
						stylers: [
							{ visibility: "off" },
						]
					}
				]
</script>
<?php
}
NOTE: Take backup of ur map.js file first if u have made any style related changes there. 
Plz let me know how it goes. 
Thanks
-Vikas