discourse/app/assets/stylesheets/common/base/request_access.scss

#request-access {
  width: 325px;
  margin: 0 auto;
  input[type="text"] {
    width: 320px;
    height: 30px;
    font-size: var(--font-up-3);
  }
  input[type="submit"] {
    font-size: var(--font-up-3);
    padding: 10px;
  }
}