For Angular 6 check the Official documentation

Note: For @angular/cli versions older than 6.0.0-beta.6 use ng set in place of ng config.

For existing projects

In an existing angular-cli project that was set up with the default css styles you will need to do a few things:

1. Change the default style extension to scss

Manually change in .angular-cli.json (Angular 5.x and older) or angular.json (Angular 6+) or run:

ng config defaults.styleExt=scss

if you get an error: Value cannot be found. use the command:

ng config schematics.@schematics/angular:component.styleext scss

(*source: Angular CLI SASS options)

2. Rename your existing .css files to .scss (i.e. styles.css and app/app.component.css)

3. Point the CLI to find styles.scss

Manually change the file extensions in apps[0].styles in angular.json

4. Point the components to find your new style files

Change the styleUrls in your components to match your new file names

For future projects

As @Serginho mentioned you can set the style extension when running the ng new command

ng new your-project-name --style=scss

If you want to set the default for all projects you create in the future run the following command:

ng config --global defaults.styleExt=scss

I just had to do this to an existing project and pieced together a bunch of information from various places, mostly from this similar stackoverflow question. Here's a set of command line actions (that worked for me on Mac OSX)

# rename all your current files
find . -name "*.css" -exec bash -c 'mv "{1%.css}".scss' - '{}' \;

# change angular cli config file to include styles.scss instead of styles.css
sed -i -e 's/styles.css/styles.scss/g' .angular-cli.json

# now we need to go thru and fix up all the places where ts files refer to .css files
# there’s probably a better way (and will only work if you don’t have multiple css files in a component)
# this is what I did
find ./src -name "*.ts" -exec sed -i -e 's/\(.*styleUrls.*\)\.css\(.*\)/\1\.scss\2/g' {} +
rm -r *.ts-e

# fix for future generates styles
ng set defaults.styleExt scss

# Add node-sass npm module

npm install node-sass --save-dev

# or if you are using yarn, like me:

yarn add node-sass --dev

In the latest version of Angular (v13), It's npm library migrating from css to scss and vice versa

ng add schematics-scss-migrate

you can change angular.json file manually:

    "schematics": {
    "@schematics/angular:component": {
      "style": "css"
    }
 ...
}

or if you just created it, simply delete it and generate a new one with css.