About file structure

Your SCSS file's location is next to zusty.scss
Project View
 ├─ scss
 │   ├─ component
 │   ├─ core
 │   ├─ elements
 │   ├─ form
 │   ├─ helpers
 │   ├─ variants
 │   └─ container.scss
 ├─ zusty.scss
 └─ yourfile.scss

 So you can easily import zusty by using
 @import "zusty";

 Learn more by reading these two docs
 zustycss.com/docs/general/customization-scss
 zustycss.com/docs/general/variables