Custom CSS and JavaScript
there are several ways to add js/css, one example:

Attaching custom javascript or css in your theme

Add a library to yourtheme.libraries.yml in your theme folder:

  yourlib:
  version: 2.6
  css:
    theme:
      css/your.css: {}
  js:
    js/your.js: {}
  dependencies:
    - core/jquery


The paths given are relative to your theme. Dependencies is optional, you can force loading of additional js like jquery.

Now to add this library to your theme go to yourtheme.info.yml and add it like this:

    libraries:
      - yourtheme/yourlib