This page explains how to add a style sheet or javascript file in to drupal8 theme.

Use the below code in THEME_NAME.libraries.yml


global-styling:
  css:
    theme:
      css/style.css: {}
      'https://fonts.googleapis.com/css?family=Fira+Sans:300,400,500,700': { type: external } 
    # lib:
    #   lib/mulihu-scrollbar/jquery.mCustomScrollbar.css: {}

  version: VERSION
  js:
    js/owl.carousel.min.js: {}

    # lib/mulihu-scrollbar/jquery.mCustomScrollbar.concat.min.js: {}
  dependencies:
    - core/jquery
    - core/drupal