Ember-power-select is distributed as an Ember-cli addon, so the only thing you need to do to install it is run the following command on your ember project directory

      $ ember install ember-power-select

If you use just vanilla css, you're good to go. Ember-power-select will add the default styles to your vendor css file.

However, if you are using SASS or LESS the addon won't do that. Instead you need to import the addon styles yourself.

      @import "ember-power-select";

When installing this through

ember install
the addon will add the snippet above automatically for you in your app.scss or app.less.

This explicit import is needed because using css preprocessors like SASS and LESS allows you to customize the appearance of ember-power-select using variables. This is easier and generates less code than overriding styles you don't like.

More information about style in Basic customization: Styles