Ember-cli-build do not inject app.css into the compiled app

I just migrated my app from ember 2.18 to 3.4. I have a css file at app/styles/app.css

Here is my ember-cli-build.js

'use strict';

const EmberApp = require('ember-cli/lib/broccoli/ember-app');

module.exports = function(defaults) {
  let app = new EmberApp(defaults, {
    // Add options here
    adminLTE: {
      plugins: ['morris','datepicker',
        'bootstrap-wysihtml5', 'daterangepicker',
        'select2', 'input-mask'
      ]
    },

    flatpickr: {
      locales: ['fr']
    },

    minifyCSS: {
        enabled: true,
        options:
        {rebaseTo: 'assets'}
      },

    'ember-bootstrap': {
      bootstrapVersion: 3,
      importBootstrapCSS: false,
      importBootstrapFont: true
    }
  });
  return app.toTree();
};

And the relevant part of index.html :

    <link integrity="" rel="stylesheet" href="assets/vendor.css">
    <link integrity="" rel="stylesheet" href="assets/marketadmin.css">
    <link integrity="" rel="stylesheet" href="css/AdminLTE.min.css">
    <link integrity="" rel="stylesheet" href="css/skins/skin-blue.min.css">

There is no CSS rules of my app.css in vendor.css or in marketadmin.css. Why?