Creating a Bartik sub-theme in Drupal 8

I have been playing with Drupal 8. At the moment I want to have an image as the header background in Bartik. For this I created a subtheme called "freex" in the following way:

Create folder freex in /themes/custom/

Create freex.info.yml in /themes/custom/freex/ containing:

name: Freex
description: Basis thema voor verenigingen
type: theme
core: 8.x
base theme: bartik

libraries:
  - freex/global-styling

Create file freex.libraries.yml in /themes/custom/freex/ containing:

global-styling:
  version: 1.0
  css:
    theme:
      css/style.css: {}

Create file in /themes/custom/freex/css/ called style.css containing:

#header {
background-color: yellow;
}

Just to see of it works... It doesn't, the header does not change background color. Any ideas as to what I am missing?