Responsive Author Style ?

I would like change avatar style for media….480px responsive design which I attached screenshots.I use custom theme on my local server.I tried some of code in stylesheet but didn’t worked.
Default code about avatar and author section

#bbpress-forums li.bbp-body div.hentry {
    display: block;
  }
  #bbpress-forums li.bbp-body div.bbp-topic-author,
  #bbpress-forums li.bbp-body div.bbp-reply-author {
    -ms-flex-align: start;
        align-items: flex-start;
    width: 100%;
    margin: 0 0 16px;
    padding: 0 0 16px;
    border-bottom: 1px solid var(--color-divider);
    box-sizing: border-box;
  }
  .vikinger-forum-reply-author {
    -ms-flex-align: start;
        align-items: flex-start;
        
  }
}