how can i change the color of that line?
Beween categories or forums or both? Can you post a more inclusive image? I don't want to waste my time answering only to find out that I'm not answering the actual CSS area you are referring to.
http://community.mybb.com/attachment.php?aid=9871
tborder?
In global.css
.tborder {
background: #111;
Change the color hex.