How do I get Grain on my site?

Hi there. I am currently working on a redesign of my homepage and I would love to add a grain/noise-overlay to my website. I use Flothemes (I‘m a photographer) and I saw another site (Flothemes as well) using that exact effect I want:
Do you guys have any idea how I can get that? I used Chrome to look at the site and found some code with „glitch“ which I think is that effect but I have no idea how to get the same on my site. I‘m a total noob, so any help would be appreciated!


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