Add Gradient background

Hello everyone,

I’d like to add a gradient background to my website.
I’ve found this [website] ( to help me create the effect I want.
As a beginner, I don’t understand how to integrate the .js file in my webflow project and I don’t know how to code the different parts HTML, CSS and JS.

Here is my site Read-Only: LINK
(how to share your site Read-Only link)

You should just be able to add a background gradient in the styles panel, no?