You can use a trick as following:
<?php header("Content-type: text/css; charset: UTF-8");?>
Also put whatever other files you need to require before beginning of your css code as:
<?php require 'config.php';?>
Then finally add this line of code which will trick NetBeans to think that you're writing inline css in php file and will work as nothing but a comment line in css file:
/*
<style>
/**/
Because otherwise if you just add <style>
to your file if will cause problems when loaded and you will have to put it, work on the file and then remove it, and then recursively put/remove it back whenever you need editing with highlighting and code completion support from NetBeans for the subjected file.
I also highly recommend emmet plugin for NetBeans; a total time saver. I hope this will be helpful for anyone out there.