Answers for "sass files"

import mixin from another file

@import "../../../theme/main.scss"

sass

npm uninstall node-sass
npm install [email protected]

sass file

Sass is a popular preprocessor scripting language that gets compiled into css. 
There are 2 file extensions:
1) .scss (newer syntax, commonly used)
2) .sass (older syntax with strict indentation)
Learn More at https://sass-lang.com/