Random Header Images?

I found a Random Header Images code here: https://www.pearsonified.com/2006/10/simple_random_header_images_for_your_blog.php

But I don’t know where to put this code because my header image code is in the style.css file whereas in the guide they state I need to add it to a .php page. I’m wondering if there is a plugin or something that can make my header images random.

I would appreciate if somebody could help me. Keep in mind that I’m a complete noob when it comes to coding, so don’t assume anything.

Thanks in advance!