discourse/plugins/chat/assets/stylesheets/common/chat-composer-button.scss

.chat-composer-button {
  box-sizing: border-box;
  display: flex;
  align-items: center;
  justify-content: center;

  &__btn {
    box-sizing: border-box;
    width: 50px;
    border: 0;
    height: 50px;
    background: none;
  }
}