Add external sass files

I want to add the weather-icons library to my iOnic 2 project.
You can find the files here:

I copied all *.scss files to app/theme/weather-icons and imported the folder to the app.core.scss file.
But i can’t use the sass variables and classes in my html files like this:

The console shows this error:

 Error: File to import not found or unreadable: ../../theme/weather-icons
        Parent style sheet: /Users/danielsogl/Documents/iOnic Projekte/UITestApp/app/pages/home/home.scss
         on line 1 of app/pages/home/home.scss
 >> @import "../../theme/weather-icons";