Mac端滚动条背景可以优化下

[ding-yufeng]

这个滚动条背景在这里显得很粗糙，可以考虑隐藏掉，跟背景融为一体：

[fancyboi999]

Hi! I submitted a PR that softens the message-area scrollbar (lighter thumb + transparent track) to blend with the background. Could you please check if this matches the desired effect? PR: #23