express-8.x-1.x-dev/modules/custom/express_back_to_top/express_back_to_top.libraries.yml

express-back-to-top:
  version: 1.x
  css:
    theme:
      css/express_back_to_top.css: {}
  js:
    js/express_back_to_top.js: {}
  dependencies:
    - core/jquery