How do I load CSS files to a custom theme? I understand this question has been asked many times. However, I still can't figure out why the CSS files of my custom Drupal 8 theme are not loading.

Edited (more info): The CSS files are not being defined at all (they do not show up in the HTML).

Here is my test.info.yml file:

name: "test"
type: theme
description: "Theme by ..."
package: custom
core: 8.x

libraries:
  - test/global-styling
  - test/global-scripts

regions:
  headline: Headline
  header: Header
  content_special: 'Content special'
  content: Content
  sidebar: Sidebar
  footer: Footer


And here is my test.libraries.yml file

global-styling:
  version: 1.0
  css:
    theme:
      css/bootstrap-4.0.0.css: {}
      css/content.css: {}
      css/footer.css: {}
      css/header.css: {}
      css/IE.css: {}
      css/lista.css: {}
      css/navigation.css: {}
      css/style.css: {}
      css/taulukko.css: {}

global-scripts:
  version: 1.0
  js:
    js/scripts.js: {}
  dependencies:
    - core/jquery


Note: The CSS files are not being defined at all (they do not show up in the HTML). So the console is not showing any errors about this issue. The css files are accessible if you manually type the path in the browser window.
After trying alot of different things I found the problem in page.html.twig file. Had misspelled one region name (a "-" instead of a "_"). Fixing the misspelled region name just got the CSS working.

Thank you all that helped me by commenting on the question.