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input.scss

$font-stack: sytem-ui, sans-serif;

@mixin font($override:null) {
    font-family: $font-stack $override;
}

.my-class {
    @include font;
}

.my-override {
    @include font(!important);
}

output.css

.my-class {
  font-family: sytem-ui, sans-serif;
}

.my-override {
  font-family: sytem-ui, sans-serif !important;
}

settings.json

{
  "sass": {
    "compiler": "dart-sass/1.26.11",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": false
}