Choose between different style for an Ember.JS application

The issue I have is that I can't find a way to 'change' the css style within my application.

The thing that I want to access is for example: I have a red theme, but I want that the user can choose an other predefined theme, like a green, or a blue theme.
The idea is that I have different app.css, how can I change between one another, I can't find method to do so. Maybe I can do it in my environnement.js?
Any tips is apreciated.
tl;dr: How to set multiple css style in our Ember.JS app?