How do I use Scss files in CodeSandbox?

Use the Parcel Bundler template instead of the create-react-app template

Also if you're using react add react and react-dom as dependencies since the Parcel Bundler template only comes with parcel as a dev dependency

https://codesandbox.io/s/q7877ov756

If you just change all instances of style.css to style.scss then it'll work