I want to have a grid display for the blog on my website

I want to have a grid display for the blog on my website and I referred to the CSS in the following WPMUDEV article:

https://premium.wpmudev.org/blog/wordpress-archive-grid/

My theme has a placeholder to add "Global CSS" and so I tried adding the code there with no success. Am I doing the right thing or the only way to do this is by editing my style.css?

Thanks
Shireen