Generated by SassMeister.com.

input.scss

.bth{
    background: red;
}

.btn-big{
    @extend .bth;
    font-size: 12px;
    
}

output.css

.bth, .btn-big {
  background: red;
}

.btn-big {
  font-size: 12px;
}

settings.json

{
  "sass": {
    "compiler": "dart-sass/1.32.12",
    "extensions": {},
    "syntax": "SCSS",
    "outputStyle": "expanded"
  },
  "autoprefixer": false
}