discourse/plugins/chat/assets/stylesheets/desktop/chat-message.scss

.chat-message-actions {
  .react-btn,
  .reply-btn,
  .bookmark-btn {
    border: 1px solid transparent;
    border-bottom-color: var(--primary-low);
    border-radius: 0;
    border-top-color: var(--primary-low);

    &:hover {
      background: var(--primary-low);
      border-color: var(--primary-low-mid);
      color: var(--primary-medium);
      z-index: 1;
    }
  }
}