Adding a custom js file to a core theme like Seven I've copied over the Seven theme to my /theme folder on Drupal 8 and I'm trying to customize it by adding in my own js file called custom.js. I added it to the seven.libraries.yml file as shown below, but I don't see it loading up on any of the pages. I even did clear the cache. Nothing shows up. I did add a custom.css file and that is showing. What could be wrong or that I'm missing? 

global-styling:
  version: VERSION
  js:
    custom.js: {}
  css:
    base:
      css/base/elements.css: {}
      css/base/typography.css: {}
      css/base/print.css: {}
    component:
      css/components/admin-list.css: {}
      css/components/content-header.css: {}
      css/components/breadcrumb.css: {}
      css/components/buttons.css: {}
Asset injector has been made specifically to carry out the functionality that you stated. Simply install the module &amp; flush cache.

Within configuration you'll see the link Asset injector and there you can select the theme to add your custom CSS/JS.

Hope that helps!