Create a custom stylesheet for Drupal 9

global-styling:
  css:
    theme:
      css/style.css: {}
      css/custom.css: {}

edit /themes/custom/subtheme/subtheme.libraries.yml

create /themes/custom/subtheme/css/custom.css

add custom css

p {
    font-family: "proxima-nova", "Helvetica Neue", Helvetica, Arial, sans-serif;
}

clear cache