Try updating the Sass node libraries included in the C1 Package - it hasn't been updated in three years and i bet something has happened in the mean time.
https://github.com/Orckestra/C1-Packages/tree/master/Composite.Web.Css.Sass/Composite.Web.Css.Sass.Package/Package/Frontend/Composite/Web/Css/Sass/sass
https://www.npmjs.com/package/node-sass
https://github.com/Orckestra/C1-Packages/tree/master/Composite.Web.Css.Sass/Composite.Web.Css.Sass.Package/Package/Frontend/Composite/Web/Css/Sass/sass
https://www.npmjs.com/package/node-sass