discourse/plugins/chat/assets/stylesheets/mobile/chat-selection-manager.scss

.chat-selection-management {
  .chat-selection-management-buttons {
    display: flex;
    flex-direction: column;
    width: 100%;

    .cancel-btn {
      margin-left: initial;
    }

    .btn {
      margin-bottom: 0.25em;
    }
  }
}