discourse/app/assets/stylesheets/common/select-box-kit/topic-notifications-button.scss

#topic-footer-buttons {
  .topic-notifications-button {
    min-width: auto;
    margin: 10px 0 15px 0;

    .btn {
      margin: 0;
    }

    .reason {
      line-height: 16px;
      margin: 0 0 0 10px;
    }
  }
}

.topic-notifications-button .topic-notifications-options {
  min-width: auto;
}

.topic-notifications-button {
  display: -webkit-inline-box;
  display: -ms-inline-flexbox;
  display: inline-flex;
  -webkit-box-pack: start;
      -ms-flex-pack: start;
          justify-content: flex-start;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  margin: 0;

  .topic-notifications-options {
    display: -webkit-inline-box;
    display: -ms-inline-flexbox;
    display: inline-flex;
  }
}