Using custom theme with ember-bootstrap add-on

First of all you need to manually install bootstrap:

$ bower install bootstrap --save

Then edit ember-cli-build.js so it looks like that:

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

module.exports = function(defaults) {
  var app = new EmberApp(defaults, {
    // If you do not use ember-bootstrap - then you can omit these lines
    'ember-bootstrap': {
            'importBootstrapCSS': false
        }
  });

  app.import(app.bowerDirectory + '/bootstrap/dist/css/bootstrap.css');
  app.import(app.bowerDirectory + '/bootstrap/dist/js/bootstrap.js');
  app.import(app.bowerDirectory + '/bootstrap/dist/fonts/glyphicons-halflings-regular.woff', {
    destDir: 'fonts'
  });

  return app.toTree();
};

Now you have fully working bootstrap. To add Celurian theme copy its bootstrap.css to vendor/ directory. Then remove original bootstrap.css from ember-cli-build.js and add theme css:

module.exports = function(defaults) {
  var app = new EmberApp(defaults, {
    // If you do not use ember-bootstrap - then you can omit these lines
    'ember-bootstrap': {
            'importBootstrapCSS': false
        }
  });

  //app.import(app.bowerDirectory + /bootstrap/dist/css/bootstrap.css');

  // The name does not matter, default bootstrap.css will work as well
  app.import('vendor/celurian.css');
  app.import(app.bowerDirectory + '/bootstrap/dist/js/bootstrap.js');
  app.import(app.bowerDirectory + '/bootstrap/dist/fonts/glyphicons-    halflings-regular.woff', {
    destDir: 'fonts'
  });

  return app.toTree();
};

So, in essence, you need to add required files to vendor directory and import them in ember-cli-build.js.
The other way would be to use SASS and just import required files from app.scss.