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Use Less to preprocess your ember-cli app's css.

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npm install --save-dev ember-cli-less


By default, this addon will compile app/styles/app.less into dist/assets/app.css. Additional options can be specified using the lessOptions config property in ember-cli-build.js:

let app = new EmberApp({
  lessOptions: {...}

Available Options:

  • paths: An array of include paths
  • sourceMap: Whether to generate source maps. Defaults to true in development and can also take an object of sub options:
  • mergeTrees: An object of the available options to pass to the internal merge trees plugin

Configuring Input/Output Paths

You can configure the input and output files using ember-cli's outputPaths option in ember-cli-build.js:

let app = new EmberApp({
  outputPaths: {
    app: {
      css: {
        'app': '/assets/my-project.css'

You can also configure multiple input/output paths to generate multiple css files:

let app = new EmberApp({
  outputPaths: {
    app: {
      css: {
        'theme-orange': '/assets/theme-orange.css',
        'theme-purple': '/assets/theme-purple.css'

Notice that you cannot specify the name of the Source Map if multiple input/output paths are used. The name gets generated from the output path (/assets/theme-orange.css -> /assets/

Usage in Addons

You can also use this to precompile less files in an addon. By default, this will compile addon/styles/addon.less into css that will be merged into the host app's css. (requires ember-cli >= 0.2.0):

  1. Install ember-cli-less in your addon's package.json under dependencies
  2. Create your addon less file at addon/styles/addon.less (or where you specify in your options)
  3. To run the addon's dummy app, be sure to create tests/dummy/app/styles/app.less if it doesn't exist
  4. To make less files available for applications that consume this addon, create app/styles/app.less in your addon and add @import 'addon/styles/addon'; to its content

To include custom css files, use @import statment in addon/styles/addon.less. For example:

// addon/styles/addon.less 
@import "bower_components/bootstrap/less/bootstrap";  // look for "bower_components/bootstrap/less/bootstrap.less" 


Using Bootstrap Less source in your app:

Install Bootstrap source:

bower install --S bootstrap

Specify the include paths in ember-cli-build.js:

let app = new EmberApp({
  lessOptions: {
    paths: [

Import into app.less:

@import 'bootstrap';

Linking source maps

When sourcemaps are enabled, you'll see the correct references to the original less files and their corresponding line numbers in Dev Tools. If you would like to link the less source files directly, you have to link them to your local filesystem in Chrome:

  1. Open dev tools > Sources tab
  2. Expand the sources pane on the left if it's not open
  3. Right-click anywhere, Add folder to workspace, add your project's folder
  4. Locate any less source file in the tree, right-click, Map to Network Resource... to create the mapping



npm i ember-cli-less

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